Aaron J. Romanowsky

Expanded Bibliography

Condensed Bibliography | Refereed Papers | Unrefereed Papers | Conference Proceedings

Refereed Papers

SPIDER - VII. The central dark matter content of bright early-type galaxies: benchmark correlations with mass, structural parameters and environment
C. Tortora, F. La Barbera, N.R. Napolitano, R.R. de Carvalho, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, submitted, arXiv:1201.2945
ADS | arXiv:1201.2945

We analyze the central dark-matter (DM) content of ~4,500 massive (M_* >~ 10¹⁰ M_Sun), low-redshift (z < 0.1), early-type galaxies (ETGs), with high-quality ugrizYJHK photometry and optical spectroscopy from SDSS and UKIDSS. We estimate the "central" fraction of DM within the K-band effective radius, R_eff. The main results of the present work are the following: (1) DM fractions increase systematically with both structural parameters (i.e. R_eff, and Sérsic index, n) and mass proxies (central velocity dispersion, stellar and dynamical mass), as in previous studies, and decrease with central stellar density. 2) All correlations involving DM fractions are caused by two fundamental ones with galaxy effective radius and central velocity dispersion. These correlations are independent of each other, so that ETGs populate a central-DM plane (DMP), i.e. a correlation among fraction of total-to-stellar mass, effective radius, and velocity dispersion, whose scatter along the total-to-stellar mass axis amounts to ~0.15 dex. (3) In general, a Chabrier IMF is favoured with respect to a bottom-heavier Salpeter IMF, as the latter produces negative (i.e. unphysical) DM fractions for more than 50% of the galaxies in our sample. For a Chabrier IMF, the DM estimates agree with LCDM toy-galaxy models based on contracted DM-halo density profiles. We also find agreement with predictions from hydrodynamical simulations. (4) The central DM content of ETGs does not depend significantly on the environment where galaxies reside, with group and field ETGs having similar DM trends.

Evidence for inhomogeneous reionization in the local Universe from metal-poor globular cluster systems
L.R. Spitler, A.J. Romanowsky, J. Diemand, J. Strader, D.A. Forbes, B. Moore, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, submitted, arXiv:1112.2213
ADS | arXiv:1112.2213

Exploiting a fundamental characteristic of galaxy assembly in the LCDM paradigm, the observed spatial biasing and kinematics of metal-poor globular star clusters are used to constrain the local reionization epoch around individual galaxies. Selecting three galaxies located in different environments, the first attempt at constraining the environmental propagation of reionization in the local Universe is carried out. The joint constraint from the three galaxies (z_reion = 10.5^+1.0_-0.9) agrees remarkably well with the latest WMAP constraint on z_reion for a simple instantaneous reionization model. More importantly, the range of z_reion alues found here are consistent with the global range of z_reion estimates from other observations. We furthermore find a 1.7-sigma indication that reionization completed in low-density environments before the intergalactic medium in high-density environments was reionized. This is consistent with certain theoretical models that predict that reionization was globally prolonged in duration, with neutral hydrogen pockets surviving in high-density environments, even after the surrounding regions were reionized. More generally, this work provides a useful constraint on the formation history of galaxy stellar halos.

Dwarfs gobbling dwarfs: A stellar tidal stream around NGC 4449 and hierarchical galaxy formation on small scales
D. Martínez-Delgado, A.J. Romanowsky, R.J. GaBany, J.A. Arnold, F. Annibali, J. Fliri, S. Zibetti, R.P. van der Marel, H.-W. Rix, T.S. Chonis, J.A. Carballo-Bello, A. Aloisi, A.V. Maccio, J. Gallego-Laborda, J.P. Brodie, M.R. Merrifield
The Astrophysical Journal Letters, in press, arXiv:1112.2154
ADS | arXiv:1112.2154
Citations: [1] [2]

A candidate diffuse stellar substructure was previously reported in the halo of the nearby dwarf starburst galaxy NGC 4449 by Karachentsev et al. We map and analyze this feature using a unique combination of deep integrated-light images from the Black Bird 0.5-meter telescope, and high-resolution wide-field images from the 8-meter Subaru telescope, which resolve the nebulosity into a stream of red giant branch stars, and confirm its physical association with NGC 4449. The properties of the stream imply a massive dwarf spheroidal progenitor, which after complete disruption will deposit an amount of stellar mass that is comparable to the existing stellar halo of the main galaxy. The ratio between luminosity or stellar-mass between the two galaxies is ~1:50, while the indirectly measured dynamical mass-ratio, when including dark matter, may be ~1:10-1:5. This system may thus represent a "stealth" merger, where an infalling satellite galaxy is nearly undetectable by conventional means, yet has a substantial dynamical influence on its host galaxy. This singular discovery also suggests that satellite accretion can play a significant role in building up the stellar halos of low-mass galaxies, and possibly in triggering their starbursts.

Testing Yukawa-like potentials from f(R)-gravity in elliptical galaxies
N.R. Napolitano, S. Capozziello, A.J. Romanowsky, M. Capaccioli, C. Tortora
The Astrophysical Journal, in press, arXiv:1201.3363
ADS | arXiv:1201.3363 | Press release

We present the first analysis of extended stellar kinematics of elliptical galaxies where a Yukawa-like correction to the Newtonian gravitational potential derived from f(R)-gravity is considered as an alternative to dark matter. In this framework, we model long-slit data and planetary nebulae data out to 7 R_eff of three galaxies with either decreasing or flat dispersion profiles. We use the corrected Newtonian potential in a dispersion-kurtosis Jeans analysis to account for the mass-anisotropy degeneracy. We find that these modified potentials are able to fit nicely all three elliptical galaxies and the anisotropy distribution is consistent with that estimated if a dark halo is considered. The parameter which measures the "strength" of the Yukawa-like correction is, on average, smaller than the one found previously in spiral galaxies and correlates both with the scale length of the Yukawa-like term and the orbital anisotropy.

The ongoing assembly of a central cluster galaxy: phase-space substructures in the halo of M87
A.J. Romanowsky, J. Strader, J.P. Brodie, J.C. Mihos, L.R. Spitler, D.A. Forbes, C. Foster, J.A. Arnold
The Astrophysical Journal, in press, arXiv:1112.3959
ADS | arXiv:1112.3959 | High-res PDF

The halos of galaxies preserve unique records of their formation histories. We carry out the first combined observational and theoretical study of phase-space halo substructure in an early-type galaxy: M87, the central galaxy in the Virgo cluster. We analyze an unprecedented wide-field, high-precision photometric and spectroscopic data set for 488 globular clusters (GCs), which includes new, large-radius Subaru/Suprime-Cam and Keck/DEIMOS observations. We find signatures of two substructures in position-velocity phase-space. One is a small, cold stream associated with a known stellar filament in the outer halo; the other is a large shell-like pattern in the inner halo that implies a massive, hitherto unrecognized accretion event. We perform extensive statistical tests and independent metallicity analyses to verify the presence and characterize the properties of these features, and to provide more general methodologies for future extragalactic studies of phase-space substructure. The cold outer stream is consistent with a dwarf galaxy accretion event, while for the inner shell there is tension between a low progenitor mass implied by the cold velocity dispersion, and a high mass from the large number of GCs, which might be resolved by a ~0.5 L^* E/S0 progenitor. We also carry out proof-of-principle numerical simulations of the accretion of smaller galaxies in an M87-like gravitational potential. These produce analogous features to the observed substructures, which should have observable lifetimes of ~1 Gyr. The shell and stream GCs together support a scenario where the extended stellar envelope of M87 has been built up by a steady rain of material that continues until the present day.

Kinematic properties of the field elliptical NGC 7507
R. Salinas, T. Richtler, L.P. Bassino, A.J. Romanowsky, Y. Schuberth
Astronomy & Astrophysics, in press, arXiv:1111.1581
ADS | arXiv:1111.1581
Citations: [1]

The dark matter halos of field elliptical galaxies are not well studied and appear controversial in the literature. There are galaxies seeming nearly devoid of dark matter and others showing clear signatures of its presence. Furthermore, the MOND prescription, which has been shown predictive power in the domain of disk galaxies, has not been investigated for isolated elliptical galaxies. We study the kinematics of the isolated elliptical NGC 7507 which has been claimed as a clear case of dark matter presence in earlytype galaxies. We obtained major and minor axis long slit spectroscopy of NGC 7507 using the Gemini South telescope and deep imaging in Kron-Cousins R and Washington C using the CTIO/MOSAIC camera. Mean velocities, velocity dispersion and higher order moments are measured out to ~90°. The galaxy, although almost circular, presents significant rotation along the minor axis and a rapidly declining velocity dispersion in both axes. The velocity dispersion profile is modeled in the context of spherical Jeans analysis. Models without DM provide an excellent representation of the data with an M/L ratio of 3.1 (R-band). The most massive NFW halo the data allows has a virial mass of only 3.9^+3.1_2.1 ×10¹¹M_Sun, although the data favor models with slight radial anisotropy which implies an even lower DM halo mass of 2.2^+2.0_1.2 ×10¹¹M_Sun. Modeling of the h₄ Gauss-Hermite coefficient is inconclusive but seems to favor the presence of some radial anisotropy. A cored logarithmic DM halo with parameters r₀ = 7 kpc and v₀ = 100 km s^-1 can also reproduce the observed velocity dispersion profile. MOND predictions overestimate the velocity dispersion. In conclusion, we cannot easily reproduce the previously found dominance of dark matter in NGC 7507 within a simple spherical model. Dark matter may be present, but only in conjunction with a strong radial anisotropy, for which there are some indications.

Stellar mass-to-light ratio gradients in galaxies: correlations with mass
C. Tortora, N.R. Napolitano, A.J. Romanowsky, Ph. Jetzer, V.F. Cardone, M. Capaccioli
Monthly Notices of the Royal Astronomical Society, Volume 418, Issue 3, December 2011, pp. 1557-1564
ADS | arXiv:1107.2918
Citations: [1]
We analyze the stellar mass-to-light ratio (M/L) gradients in a large sample of local galaxies taken from the Sloan Digital Sky Survey, spanning a wide range of stellar masses and morphological types. As suggested by the well known relationship between mass-to-light (M/L) ratios and colors, we show that M/L gradients are strongly corre- lated with colour gradients, which we trace to the effects of age variations. Stellar M/L gradients generally follow patterns of variation with stellar mass and galaxy type that were previous found for colour and metallicity gradients. In late-type galaxies M/L gradients are negative, steepening with increasing mass. In early-type galaxies M/L gradients are shallower while presenting a two-fold trend: they decrease with mass up to a characteristic mass of M_*~10^10.3M_Sun and increase at larger masses. We compare our findings with other analyses and discuss some implications for galaxy formation and for dark matter estimates.

Wide-field precision kinematics of the M87 globular cluster system
J. Strader, A.J. Romanowsky, J.P. Brodie, L.R. Spitler, M.A. Beasley, J.A. Arnold, N. Tamura, R.M. Sharples, N. Arimoto
The Astrophysical Journal Supplement Series, Volume 197, Number 2, 2011 December, 33 (49pp)
ADS | ApJS | arXiv:1110.2778
Citations: [1] [2] [3] [4] [5] [6]

We present the most extensive combined photometric and spectroscopic study to date of the enormous globular cluster (GC) system around M87, the central giant elliptical galaxy in the nearby Virgo Cluster. Using observations from DEIMOS and the Low Resolution Imaging Spectrometer at Keck, and Hectospec on the Multiple Mirror Telescope, we derive new, precise radial velocities for 451 GCs around M87, with projected radii from ~5 to 185 kpc. We combine these measurements with literature data for a total sample of 737 objects, which we use for a re-examination of the kinematics of the GC system of M87. The velocities are analyzed in the context of archival wide-field photometry and a novel Hubble Space Telescope catalog of half-light radii, which includes sizes for 344 spectroscopically confirmed clusters. We use this unique catalog to identify 18 new candidate ultracompact dwarfs and to help clarify the relationship between these objects and true GCs. We find much lower values for the outer velocity dispersion and rotation of the GC system than in earlier papers and also differ from previous work in seeing no evidence for a transition in the inner halo to a potential dominated by the Virgo Cluster, nor for a truncation of the stellar halo. We find little kinematical evidence for an intergalactic GC population. Aided by the precision of the new velocity measurements, we see significant evidence for kinematical substructure over a wide range of radii, indicating that M87 is in active assembly. A simple, scale-free analysis finds less dark matter within ~85 kpc than in other recent work, reducing the tension between X-ray and optical results. In general, out to a projected radius of ~150 kpc, our data are consistent with the notion that M87 is not dynamically coupled to the Virgo Cluster; the core of Virgo may be in the earliest stages of assembly.

The relationships among compact stellar systems: a fresh view of ultra compact dwarfs
J.P. Brodie, A.J. Romanowsky, J. Strader, D.A. Forbes
The Astronomical Journal, Volume 142, 2011 December, 199 (16pp)
ADS | AJ | arXiv:1109.5696
Citations: [1] [2] [3] [4]

We use a combined imaging and spectroscopic survey of the nearby central cluster galaxy, M87, to assemble a sample of 34 confirmed ultracompact dwarfs (UCDs) with half-light radii of >~ 10 pc measured from Hubble Space Telescope images. This doubles the existing sample in M87, making it the largest such sample for any galaxy, while extending the detection of UCDs to unprecedentedly low luminosities (MV = -9). With this expanded sample, we find no correlation between size and luminosity, in contrast to previous suggestions, and no general correlation between size and galactocentric distance. We explore the relationships between UCDs, less luminous extended clusters (including faint fuzzies), globular clusters (GCs), as well as early-type galaxies and their nuclei, assembling an extensive new catalog of sizes and luminosities for stellar systems. Most of the M87 UCDs follow a tight colormagnitude relation, offset from the metal-poor GCs. This, along with kinematical differences, demonstrates that most UCDs are a distinct population from normal GCs, and not simply a continuation to larger sizes and higher luminosities. The UCD colormagnitude trend couples closely with that for Virgo dwarf elliptical nuclei.We conclude that theM87 UCDs are predominantly stripped nuclei. The brightest and reddest UCDs may be the remnant nuclei of more massive galaxies while a subset of the faintest UCDs may be tidally limited and related to more compact star clusters. In the broader context of galaxy assembly, blue UCDs may trace halo build-up by accretion of low-mass satellites, while red UCDs may be markers of metal-rich bulge formation in larger galaxies.

Galaxies in LCDM with Halo Abundance Matching: luminosity-velocity relation, baryonic mass-velocity relation, velocity function, and clustering
S. Trujillo-Gomez, A. Klypin, J. Primack, A.J. Romanowsky
The Astrophysical Journal, Volume 742, Number 1, 2011 November 20, 16 (23pp)
ADS | ApJ | arXiv:1005.1289
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

It has long been regarded as difficult if not impossible for a cosmological model to account simultaneously for the galaxy luminosity, mass, and velocity distributions. We revisit this issue using a modern compilation of observational data along with the best available large-scale cosmological simulation of dark matter (DM). We find that the standard cosmological model, used in conjunction with halo abundance matching (HAM) and simple dynamical corrections, fits -- at least on average -- all basic statistics of galaxies with circular velocities V_circ > 80 km s^-1 calculated at a radius of ~10 kpc. Our primary observational constraint is the luminosity-velocity (LV) relation -- which generalizes the Tully-Fisher and Faber-Jackson relations in allowing all types of galaxies to be included, and provides a fundamental benchmark to be reproduced by any theory of galaxy formation. We have compiled data for a variety of galaxies ranging from dwarf irregulars to giant ellipticals. The data present a clear monotonic LV relation from ~50 km s^-1 to ~500 km s^-1, with a bend below ~80 km s^-1 and a systematic offset between late- and early-type galaxies. For comparison to theory, we employ our new LCDM "Bolshoi" simulation of DM, which has unprecedented mass and force resolution over a large cosmological volume, while using an up-to-date set of cosmological parameters. We use HAM to assign rank-ordered galaxy luminosities to the DM halos, a procedure that automatically fits the empirical luminosity function and provides a predicted LV relation that can be checked against observations. The adiabatic contraction of DM halos in response to the infall of the baryons is included as an optional model ingredient. The resulting predictions for the LV relation are in excellent agreement with the available data on both early-type and late-type galaxies for the luminosity range from M_r = -14 to M_r = -22. We also compare our predictions for the "cold" baryon mass (i.e., stars and cold gas) of galaxies as a function of circular velocity with the available observations, again finding a very good agreement. The predicted circular velocity function (VF) is also in agreement with the galaxy VF from 80 to 400 km s^-1, using the HIPASS survey for late-type galaxies and Sloan Digital Sky Survey (SDSS) for early-type galaxies. However, in accord with other recent results, we find that the DM halos with V_circ < 80 km s^-1 are much more abundant than observed galaxies with the same V_circ. Finally, we find that the two-point correlation function of bright galaxies in our model matches very well the results from the final data release of the SDSS, especially when a small amount of scatter is included in the HAM prescription.

Global properties of 'ordinary' early-type galaxies: photometry and spectroscopy of stars and globular clusters in NGC 4494
C. Foster, L.R. Spitler, A.J. Romanowsky, D.A. Forbes, V. Pota, K. Bekki, J. Strader, R.N. Proctor, J.A. Arnold, J.P. Brodie
Monthly Notices of the Royal Astronomical Society, Volume 415, Issue 4, August 2011, pp. 3393-3416
ADS | arXiv:1104.5503
Citations: [1] [2] [3] [4] [5] [6] [7]

We present a comprehensive analysis of the spatial, kinematic, and chemical properties of stars and globular clusters (GCs) in the 'ordinary' elliptical galaxy NGC 4494 using data from the Keck and Subaru telescopes. We derive galaxy surface brightness and colour profiles out to large galactocentric radii. We compare the latter to metallicities derived using the near-infrared Calcium Triplet. We obtain stellar kinematics out to ~3.5 effective radii. The latter appear flattened or elongated beyond ~1.8 effective radii in contrast to the relatively round photometric isophotes. In fact, NGC 4494 may be a flattened galaxy, possibly even an S0, seen at an inclination of ~45 degrees. We publish a catalogue of 431 GC candidates brighter than i₀ = 24 based on the photometry, of which 109 are confirmed spectroscopically and 54 have measured spectroscopic metallicities. We also report the discovery of 3 spectroscopically confirmed ultra-compact dwarfs around NGC 4494 with measured metallicities of -0.4 <~ [Fe/H] <~ -0.3. Based on their properties, we conclude that they are simply bright GCs. The metal-poor globular clusters are found to be rotating with similar amplitude as the galaxy stars, while the metal-rich globular clusters show marginal rotation. We supplement our analysis with available literature data and results. Using model predictions of galaxy formation, and a suite of merger simulations, we find that many of the observational properties of NGC 4494 may be explained by formation in a relatively recent gas-rich major merger. Complete studies of individual galaxies incorporating a range of observational avenues and methods such as the one presented here will be an invaluable tool for constraining the fine details of galaxy formation models, especially at large galactocentric radii.

The fossil record of two-phase galaxy assembly: Kinematics and metallicities in the nearest S0 galaxy
J.A. Arnold, A.J. Romanowsky, J.P. Brodie, L. Chomiuk, L.R. Spitler, J. Strader, A.J. Benson, D.A. Forbes
The Astrophysical Journal Letters, Volume 736, Number 2, 2011 August 1, L26 (5pp)
ADS | ApJL | arXiv:1106.0745
Citations: [1] [2] [3] [4] [5] [6] [7] [8]

We present a global analysis of kinematics and metallicity in the nearest S0 galaxy, NGC 3115, along with implications for its assembly history. The data include high-quality wide-field imaging from Suprime-Cam on the Subaru telescope, and multi-slit spectra of the field stars and globular clusters (GCs) obtained using Keck-DEIMOS/LRIS and Magellan-IMACS. Within two effective radii, the bulge (as traced by the stars and metal-rich GCs) is flattened and rotates rapidly (v/sigma >~ 1.5). At larger radii, the rotation declines dramatically to v/sigma~0.7, but remains well aligned with the inner regions. The radial decrease in characteristic metallicity of both the metal-rich and metal-poor GC subpopulations produces strong gradients with power-law slopes of -0.17 ± 0.04 and -0.38 ± 0.06 dex dex^-1, respectively. We argue that this pattern is not naturally explained by a binary major merger, but instead by a two-phase assembly process where the inner regions have formed in an early violent, dissipative phase, followed by the protracted growth of the outer parts via minor mergers with typical mass ratios of ~15-20:1.

An extremely luminous panchromatic outburst from the nucleus of a distant galaxy
A.J. Levan, N.R. Tanvir, S.B. Cenko, D.A. Perley, K. Wiersema, J.S. Bloom, A.S. Fruchter, A. de Ugarte Postigo, P.T. O'Brien, N. Butler, A.J. van der Horst, G. Leloudas, A.N. Morgan, K. Misra, G.C. Bower, J. Farihi, R.L. Tunnicliffe, M. Modjaz, J.M. Silverman, J. Hjorth, C. Thöne, A. Cucchiara, J.M. Castro Cerón, A.J. Castro-Tirado, J.A. Arnold, M. Bremer, J.P. Brodie, T. Carroll, M.C. Cooper, P.A. Curran, R.M. Cutri, J. Ehle, D. Forbes, J. Fynbo, J. Gorosabel, J. Graham, D.I. Hoffman, S. Guziy, P. Jakobsson, A. Kamble, T. Kerr, M.M. Kasliwal, C. Kouveliotou, D. Kocevski, N.M. Law, P.E. Nugent, E.O. Ofek, D. Poznanski, R.M. Quimby, E. Rol, A.J. Romanowsky, R. Sánchez-Ramírez, S. Schulze, N. Singh, L. van Spaandonk, R.L.C. Starling, R.G. Strom, J.C. Tello, O. Vaduvescu, P.J. Wheatley, R.A.M.J. Wijers, J.M. Winters, D. Xu
Science, Volume 333, Number 6039, 8 July 2011, pp. 199-202
ADS | arXiv:1104.3356 | Science | Science Express | BBC
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

Variable x-ray and -ray emission is characteristic of the most extreme physical processes in the universe. We present multiwavelength observations of a unique -rayselected transient detected by the Swift satellite, accompanied by bright emission across the electromagnetic spectrum, and whose properties are unlike any previously observed source. We pinpoint the event to the center of a small, star-forming galaxy at redshift z = 0.3534. Its high-energy emission has lasted much longer than any -ray burst, whereas its peak luminosity was 100 times higher than bright active galactic nuclei. The association of the outburst with the center of its host galaxy suggests that this phenomenon has its origin in a rare mechanism involving the massive black hole in the nucleus of that galaxy.

The dark halo of the Hydra I galaxy cluster: core, cusp, cosmological? Dynamics of NGC 3311 and its globular cluster system
T. Richtler, R. Salinas, I. Misgeld, M. Hilker, G.K.T. Hau, A.J. Romanowsky, Y. Schuberth, M. Spolaor
Astronomy & Astrophysics, Volume 531, July 2011, Article A119 (8pp)
ADS | arXiv:1103.2053
Citations: [1] [2] [3] [4] [5] [6] [7]

Some galaxy clusters exhibit shallow or even cored dark matter density profiles in their central regions rather than the predicted steep or cuspy profiles, conflicting with the standard understanding of dark matter. NGC 3311 is the central cD galaxy of the Hydra I cluster (Abell 1060). We use globular clusters around NGC 3311, combined with kinematical data of the galaxy itself, to investigate the dark matter distribution in the central region of Hydra I. Radial velocities of 118 bright globular clusters, based on VLT/VIMOS mask spectroscopy, are used to calculate velocity dispersions which are well defined out to 100 kpc. NGC 3311 is the most distant galaxy for which this kind of study has been performed. We also determine the velocity dispersions of the stellar component from long-slit spectroscopy of NGC 3311 acquired with VLT/FORS1 out to 20 kpc. We present a new photometric model for NGC 3311, based on deep VLT/FORS1 images in the V-band. We search for a dark halo that, in the context of a spherical Jeans model, can reproduce the kinematical data. We also compare the radial velocity distributions of globular clusters and planetary nebulae. The projected stellar velocity dispersion rises from a central low value of about 185 km s^-1 to 350 km s^-1 at a radius of 20 kpc. The globular cluster dispersion rises as well from 500 km s^-1 at 10 kpc to about 800 km s^-1 at 100 kpc, comparable to the velocity dispersion of the cluster galaxies. A dark matter halo with a core (Burkert halo) closely reproduces the velocity dispersions of stars and globular clusters simultaneously under isotropy. The central stellar velocity dispersions predicted by cosmological NFWhalos do not agree well with those observed, while the globular clusters allow a wide range of halo parameters. A suspected radial anisotropy of the stellar population found in merger simulations aggravates the disagreement with observations. A slight tangential anisotropy would enable the data to be more accurately reproduced. However, we find discrepancies with previous kinematical data that we cannot resolve, which may indicate a more complicated velocity pattern. Although one cannot conclusively demonstrate that the dark matter halo of NGC 3311 has a core rather than a cusp, a core seems to be most consistent with the present data. A more complete coverage of the velocity field and a more thorough analysis of the anisotropy is required to reach firm conclusions.

Unravelling the origins of S0 galaxies using maximum likelihood analysis of planetary nebulae kinematics
A. Cortesi, M.R. Merrifield, M. Arnaboldi, O. Gerhard, I. Martinez-Valpuesta, K. Saha, L. Coccato, S. Bamford, N.R. Napolitano, P. Das, N.G. Douglas, A.J. Romanowsky, K. Kuijken, M. Capaccioli, K.C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 414, Issue 1, June 2011, pp. 642-651
ADS | arXiv:1101.5092
Citations: [1]

To investigate the origins of S0 galaxies, we present a new method of analysing their stellar kinematics from discrete tracers such as planetary nebulae. This method involves binning the data in the radial direction so as to extract the most general possible non-parametric kinematic profiles, and using a maximum-likelihood fit within each bin in order to make full use of the information in the discrete kinematic tracers. Both disc and spheroid kinematic components are fitted, with a two-dimensional decomposition of imaging data used to attribute to each tracer a probability of membership in the separate components. Likelihood clipping also allows us to identify objects whose properties are not consistent with the adopted model, rendering the technique robust against contaminants and able to identify additional kinematic features.
The method is first tested on an N-body simulated galaxy to assess possible sources of systematic error associated with the structural and kinematic decomposition, which are found to be small. It is then applied to the S0 system NGC 1023, for which a planetary nebula catalogue has already been released and analysed by Noordermer et al. The correct inclusion of the spheroidal component allows us to show that, contrary to previous claims, the stellar kinematics of this galaxy are indistinguishable from those of a normal spiral galaxy, indicating that it may have evolved directly from such a system via gas stripping or secular evolution. The method also successfully identifies a population of outliers whose kinematics are different from those of the main galaxy; these objects can be identified with a stellar stream associated with the companion galaxy NGC 1023A.

Evidence for two phases of galaxy formation from radial trends in the globular cluster system of NGC 1407
D.A. Forbes, L.R. Spitler, J. Strader, A.J. Romanowsky, J.P. Brodie, C. Foster
Monthly Notices of the Royal Astronomical Society, Volume 413, Issue 4, June 2011, pp. 2943-2949
ADS | ADS | arXiv:1101.3575
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

Here we present the colours of individual globular clusters (GCs) around the massive elliptical galaxy NGC 1407 out to a projected galactocentric radius of 140 kpc or 17 galaxy effective radii (R_e). Such data are a proxy for the halo metallicity. We find steep, and similar, metallicity gradients of ~0.4 dex dex^-1 for both the blue (metal poor) and red (metal rich) GC subpopulations within 5-8.5 R_e (40-70 kpc). At larger radii the mean GC colours (metallicity) are constant. A similar behaviour is seen in a wide-field study of M87's GC system, and in our own Galaxy. We interpret these radial metallicity trends to indicate an inner region formed by early in situ dissipative processes and an outer halo formed by the ongoing accretion of low-mass galaxies and their GCs. These results provide observational support for the model of galaxy formation whereby massive galaxies form inside-out in two phases. We have also searched the literature for other massive early-type galaxies with reported GC metallicity gradients in their inner regions. No obvious correlation with galaxy mass or environment is found but the sample is currently small.

Optical and near-infrared velocity dispersions of early-type galaxies
J. Vanderbeke, M. Baes, A.J. Romanowsky, L. Schmidtobreick
Monthly Notices of the Royal Astronomical Society, Volume 412, Issue 3, April 2011, pp. 2017-2025
ADS | arXiv:1011.3665

We have carried out a systematic, homogeneous comparison of optical and near-infrared dispersions. Our magnitude-limited sample of early-type galaxies (ETGs) in the Fornax cluster comprises 11 elliptical and 11 lenticular galaxies more luminous than M_B=-17. We were able to determine the central dispersions based on the near-infrared CO absorption band head for 19 of those galaxies. The velocity dispersions range from less than 70 km s^-1 to over 400 km s^-1. We compare our near-infrared velocity dispersions to the optical dispersions measured by Kuntschner. Contrary to previous studies, we find a one-to-one correspondence with a median fractional difference of 6.4 per cent. We examine the correlation between the relative dust mass and the fractional difference of the velocity dispersions, but find no significant trend. Our results suggest that early-type galaxies are largely optically thin, which is consistent with recent Herschel observations.

Crazy heart: kinematics of the "star pile" in Abell 545
R. Salinas, T. Richtler, M.J. West, A.J. Romanowsky, E. Lloyd-Davies, Y. Schuberth
Astronomy & Astrophysics, Volume 528, April 2011, Article A61 (9pp)
ADS | A&A | arXiv:1102.0038
Citations: [1]

We study the structure and internal kinematics of the "star pile" in Abell 545 -- a low surface brightness structure lying in the center of the cluster.We have obtained deep long-slit spectroscopy of the star pile using VLT/FORS2 and Gemini/GMOS, which is analyzed in conjunction with deep multiband CFHT/MEGACAM imaging. As presented in a previous study the star pile has a flat luminosity profile and its color is consistent with the outer parts of elliptical galaxies. Its velocity map is irregular, with parts being seemingly associated with an embedded nucleus, and others which have significant velocity offsets to the cluster systemic velocity with no clear kinematical connection to any of the surrounding galaxies. This would make the star pile a dynamically defined stellar intra-cluster component. The complicated pattern in velocity and velocity dispersions casts doubts on the adequacy of using the whole star pile as a dynamical test for the innermost dark matter profile of the cluster. This status is fulfilled only by the nucleus and its nearest surroundings which lie at the center of the cluster velocity distribution.

The PN.S Elliptical Galaxy Survey: a standard LCDM halo around NGC 4374?
N.R. Napolitano,A.J. Romanowsky, M. Capaccioli, N.G. Douglas, M. Arnaboldi, L. Coccato, O. Gerhard, K. Kuijken, M.R. Merrifield, S.P. Bamford, A. Cortesi, P. Das, K.C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 411, Issue 3, March 2011, pp. 2035-2053
ADS | arXiv:1010.1533
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

As part of our current programme to test LCDM predictions for dark matter (DM) haloes using extended kinematical observations of early-type galaxies, we present a dynamical analysis of the bright elliptical galaxy NGC 4374 (M84) based on ~450 Planetary Nebulae (PNe) velocities from the PN.Spectrograph, along with extended long-slit stellar kinematics. This is the first such analysis of a galaxy from our survey with a radially constant velocity dispersion profile. We find that the spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The velocity kurtosis is consistent with zero at almost all radii. We construct a series of Jeans models, fitting both velocity dispersion and kurtosis to help break the mass-anisotropy degeneracy. Our mass models include DM halos either with shallow cores or with central cusps as predicted by cosmological simulations - along with the novel introduction in this context of adiabatic halo contraction from baryon infall. Both classes of models confirm a very massive dark halo around NGC 4374, demonstrating that PN kinematics data are well able to detect such haloes when present. Considering the default cosmological mass model, we confirm earlier suggestions that bright galaxies tend to have halo concentrations higher than LCDM predictions, but this is found to be solved if either a Salpeter IMF or adiabatic contraction with a Kroupa IMF is assumed. Thus for the first time a case is found where the PN dynamics may well be consistent with a standard dark matter halo. A cored halo can also fit the data, and prefers a stellar mass consistent with a Salpeter IMF. The less dramatic dark matter content found in lower-luminosity "ordinary" ellipticals suggests a bimodality in the halo properties which may be produced by divergent baryonic effects during their assembly histories.

Star clusters in M31: Old clusters with bar kinematics
H. Morrison, N. Caldwell, R.P. Schiavon, E. Athanassoula, A.J. Romanowsky, P. Harding
The Astrophysical Journal Letters, Volume 726, Number 1, 2011 January 1, L9 (4pp)
ADS | ApJL | arXiv:1011.2232
Citations: [1] [2]

We analyze our accurate kinematical data for the old clusters in the inner regions of M31. These velocities are based on high signal-to-noise Hectospec data. The data are well suited for analysis of M31's inner regions because we took particular care to correct for contamination by unresolved field stars from the disk and bulge in the fibers. The metal-poor clusters show kinematics that are compatible with a pressure-supported spheroid. The kinematics of metal-rich clusters, however, argue for a disk population. In particular the innermost region (inside 2 kpc) shows the kinematics of the x₂ family of bar periodic orbits, arguing for the existence of an inner Lindblad resonance in M31.

Central dark matter trends in early-type galaxies from strong lensing, dynamics and stellar populations
C. Tortora, N.R. Napolitano, A.J. Romanowsky, P. Jetzer
The Astrophysical Journal Letters, Volume 721, Number 1, 2010 September 20, L1-L5
ADS | ApJL | arXiv:1007.3988
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

We analyze the correlations between central dark matter (DM) content of early-type galaxies and their sizes and ages, using a sample of intermediate-redshift (z ~ 0.2) gravitational lenses from the SLACS survey, and by comparing them to a larger sample of z ~ 0 galaxies. We decompose the deprojected galaxy masses into DM and stellar components using combinations of strong lensing, stellar dynamics, and stellar populations modeling. For a given stellar mass, we find that for galaxies with larger sizes, the DM fraction increases and the mean DM density decreases, consistently with the cuspy halos expected in cosmological formation scenarios. The DM fraction also decreases with stellar age, which can be partially explained by the inverse correlation between size and age. The residual trend may point to systematic dependencies on formation epoch of halo contraction or stellar initial mass functions. These results are in agreement with recent findings based on local galaxies by Napolitano et al. and suggest negligible evidence of galaxy evolution over the last ~ 2.5 Gyr other than passive stellar aging.

The central dark matter content of early-type galaxies: scaling relations and connections with star formation histories
N.R. Napolitano, A.J. Romanowsky, C. Tortora
Monthly Notices of the Royal Astronomical Society, Volume 405, Issue 4, July 2010, pp. 2351-2371
ADS | arXiv:1003.1716
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31]

We examine correlations between the masses, sizes, and star formation histories for a large sample of low-redshift early-type galaxies, using a simple suite of dynamical and stellar populations models. We confirm an anti-correlation between size and stellar age, and survey for trends with the central content of dark matter (DM). An average relation between central DM density and galaxy size of <rho_DM> ~ R_eff^-2 provides the first clear indication of cuspy DM haloes in these galaxies -- akin to standard LCDM haloes that have undergone adiabatic contraction. The DM density scales with galaxy mass as expected, deviating from suggestions of a universal halo profile for dwarf and late-type galaxies. We introduce a new fundamental constraint on galaxy formation by finding that the central DM fraction decreases with stellar age. This result is only partially explained by the size-age dependencies, and the residual trend is in the opposite direction to basic DM halo expectations. Therefore we suggest that there may be a connection between age and halo contraction, and that galaxies forming earlier had stronger baryonic feedback which expanded their haloes, or else lumpier baryonic accretion that avoided halo contraction. An alternative explanation is a lighter initial mass function for older stellar populations.

The globular cluster system of NGC 1399. V. Dynamics of the cluster system out to 80 kpc
Y. Schuberth, T. Richtler, M. Hilker, B. Dirsch, L.P. Bassino, A.J. Romanowsky, L. Infante
Astronomy & Astrophysics, Volume 513, April 2010, Article A52
ADS | A&A | arXiv:0911.0420
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

Globular clusters (GCs) are tracers of the gravitational potential of their host galaxies. Moreover, their kinematic properties may provide clues for understanding the formation of GC systems and their host galaxies. We use the largest set of GC velocities obtained so far of any elliptical galaxy to revise and extend the previous investigations (Richtler et al. 2004) of the dynamics of NGC 1399, the central dominant galaxy of the nearby Fornax cluster of galaxies. The GC velocities are used to study the kinematics, their relation with population properties, and the dark matter halo of NGC 1399. We have obtained 477 new medium-resolution spectra (of these, 292 are spectra from 265 individual GCs, 241 of which are not in the previous data set). with the VLT FORS 2 and Gemini South GMOS multi-object spectrographs. We revise velocities for the old spectra and measure velocities for the new spectra, using the same templates to obtain an homogeneously treated data set. Our entire sample now comprises velocities for almost 700 GCs with projected galactocentric radii between 6 and 100 kpc. In addition, we use velocities of GCs at larger distances published by Bergond et al. (2007). Combining the kinematic data with wide-field photometric Washington data, we study the kinematics of the metal-poor and metal-rich subpopulations. We discuss in detail the velocity dispersions of subsamples and perform spherical Jeans modelling. The most important results are: The red GCs resemble the stellar field population of NGC 1399 in the region of overlap. The blue GCs behave kinematically more erratic. Both subpopulations are kinematically distinct and do not show a smooth transition. It is not possible to find a common dark halo which reproduces simultaneously the properties of both red and blue GCs. Some velocities of blue GCs are only to be explained by orbits with very large apogalactic distances, thus indicating a contamination with GCs which belong to the entire Fornax cluster rather than to NGC1399. Also, stripped GCs from nearby elliptical galaxies, particularly NGC1404, may contaminate the blue sample. We argue in favour of a scenario in which the majority of the blue cluster population has been accreted during the assembly of the Fornax cluster. The red cluster population shares the dynamical history of the galaxy itself. Therefore we recommend to use a dark halo based on the red GCs alone. The dark halo which fits best is marginally less massive than the halo quoted by Richtler et al. (2004). The comparison with X-ray analyses is satisfactory in the inner regions, but without showing evidence for a transition from a galaxy to a cluster halo, as suggested by X-ray work.

Probing the 2D kinematic structure of early-type galaxies out to three effective radii
R.N. Proctor, D.A. Forbes, A.J. Romanowsky, J.P. Brodie, J. Strader, M. Spolaor, J.T. Mendel, L. Spitler
Monthly Notices of the Royal Astronomical Society, Volume 398, Issue 1, September 2009, pp. 91-108
ADS | arXiv:0905.4424
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

We detail an innovative new technique for measuring the two-dimensional (2D) velocity moments (rotation velocity, velocity dispersion and Gauss-Hermite coefficients h₃ and h₄) of the stellar populations of galaxy haloes using spectra from Keck DEIMOS (Deep Imaging Multi-Object Spectrograph) multi-object spectroscopic observations. The data are used to reconstruct 2D rotation velocity maps.
Here we present data for five nearby early-type galaxies to ~three effective radii. We provide significant insights into the global kinematic structure of these galaxies, and challenge the accepted morphological classification in several cases. We show that between one and three effective radii the velocity dispersion declines very slowly, if at all, in all five galaxies. For the two galaxies with velocity dispersion profiles available from planetary nebulae data we find very good agreement with our stellar profiles. We find a variety of rotation profiles beyond one effective radius, i.e rotation speed remaining constant, decreasing and increasing with radius. These results are of particular importance to studies which attempt to classify galaxies by their kinematic structure within one effective radius, such as the recent definition of fast- and slow- rotator classes by the Spectrographic Areal Unit for Research on Optical Nebulae project. Our data suggest that the rotator class may change when larger galacto-centric radii are probed. This has important implications for dynamical modelling of early-type galaxies. The data from this study are available on-line.

Central mass-to-light ratios and dark matter fractions in early-type galaxies
C. Tortora, N.R. Napolitano, A.J. Romanowsky, M. Capaccioli, G. Covone
Monthly Notices of the Royal Astronomical Society, Volume 396, Issue 2, June 2009, pp. 1132-1150
MNRAS | ADS | arXiv:0901.3781
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43]

Dynamical studies of local elliptical galaxies and the Fundamental Plane point to a strong dependence of the total mass-to-light ratio (M/L) on luminosity with a relation of the form M/L \propto L^\gamma. The `tilt' \gamma may be caused by various factors, including stellar population properties (metallicity, age and star formation history), initial mass function, rotational support, luminosity profile non-homology and dark matter (DM) fraction. We evaluate the impact of all these factors using a large uniform dataset of local early-type galaxies from Prugniel & Simien. We take particular care in estimating the stellar masses, using a general star formation history, and comparing different population synthesis models. We find that the stellar M/L contributes little to the tilt. We estimate the total M/L using simple Jeans dynamical models, and find that adopting accurate luminosity profiles is important but does not remove the need for an additional tilt component, which we ascribe to DM. We survey trends of the DM fraction within one effective radius, finding it to be roughly constant for galaxies fainter than M_B ~ -20.5, and increasing with luminosity for the brighter galaxies; we detect no significant differences among S0s and fast- and slow-rotating ellipticals. We construct simplified cosmological mass models and find general consistency, where the DM transition point is caused by a change in the relation between luminosity and effective radius. A more refined model with varying galaxy star formation efficiency suggests a transition from total mass profiles (including DM) of faint galaxies distributed similarly to the light, to near-isothermal profiles for the bright galaxies. These conclusions are sensitive to various systematic uncertainties which we investigate in detail, but are consistent with the results of dynamics studies at larger radii.

Mapping the dark side with DEIMOS: globular clusters, X-ray gas, and dark matter in the NGC 1407 group
A.J. Romanowsky, J. Strader, L.R. Spitler, R. Johnson, J.P. Brodie, D.A. Forbes, T. Ponman
The Astronomical Journal, Volume 137, 2009 June, pp. 4956-4987
AJ | ADS | PDF | arXiv:0809.2088
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13[ [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40]

NGC 1407 is the central elliptical in a nearby evolved group of galaxies apparently destined to become a galaxy cluster core. We use the kinematics of globular clusters (GCs) to probe the dynamics and mass profile of the group's center, out to a radius of 60 kpc (~10 galaxy effective radii) -- the most extended data set to date around an early-type galaxy. This sample consists of 172 GC line-of-sight velocities, most of them newly obtained using Keck/DEIMOS, with a few additional objects identified as dwarf-globular transition objects or as intragroup GCs. We find weak rotation for the outer parts of the GC system (v/sigma ~ 0.2), with a rotational misalignment between the metal-poor and metal-rich GCs. The velocity dispersion profile declines rapidly to a radius of ~20 kpc, and then becomes flat or rising to ~60 kpc. There is evidence that the GC orbits have a tangential bias that is strongest for the metal-poor GCs -- in possible contradiction to theoretical expectations. We construct cosmologically-motivated galaxy+dark halo dynamical models and infer a total mass within 60 kpc of 3x10¹² M_Sun, which extrapolates to a virial mass of ~6x10¹³ M_Sun for a typical lambda cold dark matter (LCDM) halo -- in agreement with results from kinematics of the group galaxies. We present an independent Chandra-based analysis, whose relatively high mass at ~20 kpc disagrees strongly with the GC-based result unless the GCs are assumed to have a peculiar orbit distribution, and we therefore discuss more generally some comparisons between X-ray and optical results. The group's B-band mass-to-light ratio of ~800 (uncertain by a factor of ~2) in Solar units is extreme even for a rich galaxy cluster, much less a poor group -- placing it among the most dark matter (DM) dominated systems in the universe, and also suggesting a massive reservoir of baryons lurking in an unseen phase, in addition to the nonbaryonic DM. We compare the kinematical and mass properties of the NGC 1407 group to other nearby groups and clusters, and discuss some implications of this system for structure formation.

Dearth of dark matter or massive dark halo? Mass-shape-anisotropy degeneracies revealed by NMAGIC dynamical models of the elliptical galaxy NGC 3379
F. De Lorenzi, O. Gerhard, L. Coccato, M. Arnaboldi, M. Capaccioli, N.G. Douglas, K.C. Freeman, K. Kuijken, M.R. Merrifield, N.R. Napolitano, E. Noordermeer, A.J. Romanowsky, V.P. Debattista
Monthly Notices of the Royal Astronomical Society, Volume 395, Issue 1, May 2009, pp. 76-96
ADS | arXiv:0804.3350
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]

Recent results from the Planetary Nebula Spectrograph (PN.S) survey have revealed a rapidly falling velocity dispersion profile in the nearby elliptical galaxy NGC 3379, casting doubts on whether this intermediate-luminosity galaxy has the kind of dark matter halo expected in LCDM cosmology. We present a detailed dynamical study of this galaxy, combining ground based long-slit spectroscopy, integral-field data from the SAURON instrument, and PN.S data reaching to more than seven effective radii.
We construct dynamical models with the flexible chi²-made-to-measure particle method implemented in the NMAGIC code. We fit spherical and axisymmetric models to the photometric and combined kinematic data, in a sequence of gravitational potentials whose circular velocity curves at large radii vary between a near-Keplerian decline and the nearly flat shapes generated by massive halos.
Assuming spherical symmetry we find that the data are consistent both with near-isotropic systems dominated by the stellar mass, and with models in massive halos with strongly radially anisotropic outer parts (beta gsim

0.8 at 7R_e). Formal likelihood limits would exclude (at 1 sigma) the model with stars only, as well as halo models with v_circ(7R_e) gsim

250 km s^-1. A sequence of more realistic axisymmetric models of different inclinations and a small number of triaxial tests confirm the spherical results. All valid models fitting all the data are dynamically stable over Gyrs, including the most anisotropic ones.
Overall the kinematic data for NGC 3379 out to 7R_e are consistent with a range of mass distributions in this galaxy. NGC 3379 may well have a dark matter halo as expected by recent merger models within LCDM cosmology, provided its outer envelope is strongly radially anisotropic.

Kinematic properties of early-type galaxy haloes using planetary nebulae
L. Coccato, O. Gerhard, M. Arnaboldi, P. Das, N.G. Douglas, K. Kuijken, M.R. Merrifield, N.R. Napolitano, E. Noordermeer, A.J. Romanowsky, M. Capaccioli M., A. Cortesi, F. De Lorenzi, K.C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 394, Issue 3, April 2009, pp. 1249-1283
ADS | arXiv:0811.3203
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45]

We present new planetary nebulae (PNe) positions, radial velocities, and magnitudes for 6 early-type galaxies obtained with the Planetary Nebulae Spectrograph, their two-dimensional velocity and velocity dispersion fields. We extend this study to include an additional 10 early-type galaxies with PNe radial velocity measurements available from the literature, to obtain a broader description of the outer-halo kinematics in early-type galaxies. These data extend the information derived from stellar kinematics to typically up to ~8 R_e. The combination of photometry, stellar and PNe kinematics shows: i) good agreement between the PNe number density and the stellar surface brightness in the region where the two data sets overlap; ii) good agreement between PNe and stellar kinematics; iii) that the mean rms velocity profiles fall into two groups: with of the galaxies characterized by slowly decreasing profiles and the remainder having steeply falling profiles; iv) a larger variety of velocity dispersion profiles; v) that twists and misalignments in the velocity fields are more frequent at large radii, including some fast rotators; vi) that outer haloes are characterised by more complex radial profiles of the specific angular momentum-related lambda_R parameter than observed within 1 R_e; vii) that many objects are more rotationally dominated at large radii than in their central parts; and viii) that the halo kinematics are correlated with other galaxy properties, such as total luminosity, isophotal shape, total stellar mass, V/sigma, and alpha parameter, with a clear separation between fast and slow rotators.

The Planetary Nebula Spectrograph elliptical galaxy survey: the dark matter in NGC 4494
N.R. Napolitano, A.J. Romanowsky, L. Coccato, M. Capaccioli, N.G. Douglas, E. Noordermeer, O. Gerhard, M. Arnaboldi, F. De Lorenzi, K. Kuijken, M.R. Merrifield, E. O'Sullivan, A. Cortesi, P. Das, K.C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 393, Issue 2, February 2009, pp. 329-353
ADS | MNRAS | arXiv:0810.1291
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41]

We present new Planetary Nebula Spectrograph observations of the ordinary elliptical galaxy NGC 4494, resulting in positions and velocities of 255 PNe out to 7 effective radii (25 kpc). We also present new wide-field surface photometry from MMT/Megacam, and long-slit stellar kinematics from VLT/FORS2. The spatial and kinematical distributions of the PNe agree with the field stars in the region of overlap. The mean rotation is relatively low, with a possible kinematic axis twist outside 1 R_e. The velocity dispersion profile declines with radius, though not very steeply, down to ~70 km/s at the last data point. We have constructed spherical dynamical models of the system, including Jeans analyses with multi-component LCDM-motivated galaxies as well as logarithmic potentials. These models include special attention to orbital anisotropy, which we constrain using fourth-order velocity moments. Given several different sets of modelling methods and assumptions, we find consistent results for the mass profile within the radial range constrained by the data. Some dark matter (DM) is required by the data; our best-fit solution has a radially anisotropic stellar halo, a plausible stellar mass-to-light ratio, and a DM halo with an unexpectedly low central density. We find that this result does not substantially change with a flattened axisymmetric model. Taken together with other results for galaxy halo masses, we find suggestions for a puzzling pattern wherein most intermediate-luminosity galaxies have very low concentration halos, while some high-mass ellipticals have very high concentrations. We discuss some possible implications of these results for DM and galaxy formation.

Testing the nature of S0 galaxies using planetary nebula kinematics in NGC 1023
E. Noordermeer, M.R. Merrifield, L. Coccato, M. Arnaboldi, M. Capaccioli, N.G. Douglas, K.C. Freeman, O. Gerhard, K. Kuijken, F. De Lorenzi, N.R. Napolitano, A.J. Romanowsky
Monthly Notices of the Royal Astronomical Society, Volume 384, Issue 3, March 2008, pp. 943-952
MNRAS | ADS | arXiv:0712.0859
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]

We investigate the manner in which lenticular galaxies are formed by studying their stellar kinematics: an S0 formed from a fading spiral galaxy should display similar cold outer disc kinematics to its progenitor, while an S0 formed in a minor merger should be more dominated by random motions. In a pilot study to attempt to distinguish between these scenarios, we have measured the planetary nebula (PN) kinematics of the nearby S0 system NGC 1023. Using the Planetary Nebula Spectrograph, we have detected and measured the line-of-sight velocities of 204 candidate PNe in the field of this galaxy. Out to intermediate radii, the system displays the kinematics of a normal rotationally-supported disc system. After correction of its rotational velocities for asymmetric drift, the galaxy lies just below the spiral galaxy Tully-Fisher relation, as one would expect for a fading system. However, at larger radii the kinematics undergo a gradual but major transition to random motion with little rotation. This transition does not seem to reflect a change in the viewing geometry or the presence of a distinct halo component, since the number counts of PNe follow the same simple exponential decline as the stellar continuum with the same projected disc ellipticity out to large radii. The galaxy's small companion, NGC 1023A, does not seem to be large enough to have caused the observed modification either. This combination of properties would seem to indicate a complex evolutionary history in either the transition to form an S0 or in the past life of the spiral galaxy from which the S0 formed. More data sets of this type from both spirals and S0s are needed in order to definitively determine the relationship between these types of system.

The dark matter halo of NGC 1399 - CDM or MOND?
T. Richtler, Y. Schuberth, M. Hilker, B. Dirsch, L. Bassino, A.J. Romanowsky
Astronomy & Astrophysics, Volume 478, Number 2, February I 2008, pp. L23-L26
A&A | ADS | arXiv:0711.4077
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18[ [19] [20] [21] [22] [23] [24]

Central galaxies in galaxy clusters may be key discriminants in the competition between the cold dark matter (CDM) paradigm and modified Newtonian dynamics (MOND). We investigate the dark halo of NGC 1399, the central galaxy of the Fornax cluster, out to a galactocentric distance of 80 kpc. The data base consists of 656 radial velocities of globular clusters obtained with MXU/VLT and GMOS/Gemini, which is the largest sample so far for any galaxy. We performed a Jeans analysis for a non-rotating isotropic model. An NFW halo with the parameters r_s = 50 kpc and rho_s = 0.0065 M_Sun/pc³ provides a good description of our data, fitting well to the X-ray mass. More massive halos are also permitted that agree with the mass of the Fornax cluster as derived from galaxy velocities. We compare this halo with the expected MOND models under isotropy and find that additional dark matter on the order of the stellar mass is needed to get agreement. A fully radial infinite globular cluster system would be needed to change this conclusion. Regarding CDM, we cannot draw firm conclusions. To really constrain a cluster wide halo, more data covering a larger radius are necessary. The MOND result appears as a small-scale variant of the finding that MOND in galaxy clusters still needs dark matter.

The star pile in Abell 545
R. Salinas, T. Richtler, A.J. Romanowsky, M.J. West, Y. Schuberth
Astronomy & Astrophysics, Volume 475, Number 2, November IV 2007, pp. 507-512
A&A | ADS | arXiv:0709.3633
Citations: [1] [2] [3]

Struble (1988, ApJ, 330, L25) found what appeared to be a cD halo without cD galaxy in the center of the galaxy cluster Abell 545. This remarkable feature has been passed almost unnoticed for nearly twenty years. Our goal is to review Struble's claim by providing a first (preliminary) photometric and spectroscopic analysis of this "star pile". Based on archival VLT-images and long-slit spectra obtained with Gemini-GMOS, we describe the photometric structure and measure the redshift of the star pile and of the central galaxy. The star pile is indeed associated with Abell 545. Its velocity is higher by about 1300 km/s than that of the central object. The spectra indicate an old, presumably metal-rich population. Its brightness profile is much shallower than that of typical cD-galaxies. The formation history and the dynamical status of the star pile remain elusive, until high S/N spectra and a dynamical analysis of the galaxy cluster itself become available. We suggest that the star pile might provide an interesting test of the Cold Dark Matter paradigm.

The PN.S Elliptical Galaxy Survey: data reduction, planetary nebula catalog, and basic dynamics for NGC 3379
N.G. Douglas, N.R. Napolitano, A.J. Romanowsky, L. Coccato, K. Kuijken, M.R. Merrifield, M. Arnaboldi, O. Gerhard, K.C. Freeman, H.R. Merrett, E. Noordermeer, M. Capaccioli
The Astrophysical Journal, Volume 664, 20 July 2007, pp. 257-276
ApJ | ADS | astro-ph/0703047
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48]

We present results from Planetary Nebula Spectrograph (PN.S) observations of the elliptical galaxy NGC 3379 and a description of the data reduction pipeline. We detected 214 planetary nebulae of which 191 are ascribed to NGC 3379, and 23 to the companion galaxy NGC 3384. Comparison with data from the literature show that the PN.S velocities have an internal error of <20km/s and a possible offset of similar magnitude. We present the results of kinematic modeling and show that the PN kinematics are consistent with absorption-line data in the region where they overlap. The resulting combined kinematic data set, running from the center of NGC 3379 out to more than seven effective radii (R_eff), reveals a mean rotation velocity that is small compared to the random velocities, and a dispersion profile that declines rapidly with radius. From a series of Jeans dynamical models we find the B-band mass-to-light ratio inside 5 R_eff to be 8 to 12 in solar units, and the dark matter fraction inside this radius to be less than 40%. We compare these and other results of dynamical analysis with those of dark-matter-dominated merger simulations, finding that significant discrepancies remain, reiterating the question of whether NGC 3379 has the kind of dark matter halo that the current LambdaCDM paradigm requires.

The Araucaria Project. An accurate distance to NGC 6822 from near-infrared photometry of Cepheid variables
W. Gieren, G. Pietrzynski, J. Nalewajko, I. Soszynski, F. Bresolin, R.-P. Kudritzki, D. Minniti, A. Romanowsky
The Astrophysical Journal, Volume 647, 20 August 2006, pp. 1056-1064
ADS | ApJ | astro-ph/0605231
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25]

We have measured near-infrared magnitudes in the J and K bands for 56 Cepheid variables in the Local Group galaxy NGC 6822 with well-determined periods and optical light curves in the V and I bands. Using the template light-curve approach of Soszyński and coworkers, accurate mean magnitudes were obtained from these data, which allowed us to determine with unprecedented accuracy the distance to NGC 6822 from a multiwavelength period-luminosity solution in the VIJK bands. From our data, we obtain a distance to NGC 6822 of (m - M)₀ = 23.312 ± 0.021 (random error) mag, with an additional systematic uncertainty of ~3%. This distance value is tied to an assumed LMC distance modulus of 18.50. From our multiwavelength approach, we find for the total (average) reddening to the NGC 6822 Cepheids E(B - V) = 0.356 ± 0.013 mag, which is in excellent agreement with a previous determination by McGonegal and coworkers from near-infrared photometry and implies significant internal reddening of the Cepheids in NGC 6822. Our present, definitive distance determination of NGC 6822 from Cepheids agrees within 2% with the previous distance we had derived from optical photometry alone, but has significantly reduced error bars. Our Cepheid distance to NGC 6822 is in excellent agreement with the recent independent determination of Cioni & Habing from the I-band magnitude of the tip of the red giant branch. It also agrees well, within the errors, with the early determination of McGonegal et al. (1983) from random-phase H-band photometry of nine Cepheids.

A deep kinematic survey of planetary nebulae in the Andromeda Galaxy using the Planetary Nebula Spectrograph
H. R. Merrett, M. R. Merrifield, N. G. Douglas, K. Kuijken, A. J. Romanowsky, N. R. Napolitano, M. Arnaboldi, M. Capaccioli, K. C. Freeman, O. Gerhard, L. Coccato, D. Carter, N. W. Evans, M. I. Wilkinson, C. Halliday, T. J. Bridges
Monthly Notices of the Royal Astronomical Society, Volume 369, Issue 1, June 2006, pp. 120-142
ADS | MNRAS | astro-ph/0603125
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]

We present a catalogue of positions, magnitudes and velocities for 3300 emission-line objects found by the Planetary Nebula Spectrograph in a survey of the Andromeda Galaxy, M31. Of these objects, 2615 are found likely to be planetary nebulae (PNe) associated with M31. Initial results from this survey include: the likely non-existence of Andromeda VIII; a universal PN luminosity function, with the exception of a small amount of obscuration, and a small offset in normalization between bulge and disk components; very faint kinematically-selected photometry implying no cut-off in the disk to beyond 4 scalelengths and no halo population in excess of the bulge out to 10 effective bulge radii; disk kinematics that show significant dispersion and asymmetric drift out to large radii, consistent with a warm flaring disk; and no sign of any variation in kinematics with PN luminosity, suggesting that PNe arise from a fairly uniform population of old stars.

Planetary nebula velocities in the disk and bulge of M31
C. Halliday, D. Carter, T. J. Bridges, Z. C. Jackson, M. I. Wilkinson, D. P. Quinn, N. W. Evans, N. G. Douglas, H. R. Merrett, M. R. Merrifield, A. J. Romanowsky, K. Kuijken, M. J. Irwin
Monthly Notices of the Royal Astronomical Society, Volume 369, Issue 1, June 2006, pp. 97-119
ADS1 | ADS2 | MNRAS | astro-ph/0603706
Citations: [1] [2] [3] [4] [5] [6] [7]

We present radial velocities for a sample of 723 planetary nebulae (PNe) in the disk and bulge of M31, measured using the WYFFOS fibre spectrograph on the William Herschel telescope. Velocities are determined using the [OIII] 5007 Angstrom emission line. Rotation and velocity dispersion are measured to a radius of 50 arcminutes (11.5 kpc), the first stellar rotation curve and velocity dispersion profile for M31 to such a radius. Our kinematics are consistent with rotational support at radii well beyond the bulge effective radius of 1.4kpc, although our data beyond a radius of 5kpc are limited. We present tentative evidence for kinematic substructure in the bulge of M31 to be studied fully in a later work. This paper is part of an ongoing project to constrain the total mass, mass distribution and velocity anisotropy of the disk, bulge and halo of M31.

Wide-field kinematics of globular clusters in the Leo I group
G. Bergond, S. E. Zepf, A. J. Romanowsky, R. M. Sharples, K. L. Rhode
Astronomy & Astrophysics, Volume 448, Number 1, March II 2006, pp. 155-164
astro-ph/0511492 | ADS | A&A
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29]

We present wide-field spectroscopy of globular clusters around the Leo I group galaxies NGC 3379 and NGC 3384 using the FLAMES multi-fibre instrument at the VLT. We obtain accurate radial velocities for 42 globular clusters (GCs) in total, 30 for GCs around the elliptical NGC 3379, eight around the lenticular NGC 3384, and four which may be associated with either galaxy. These data are notable for their large radial range extending from 0.7' to 14.5' (2 to 42 kpc) from the centre of NGC 3379, and small velocity uncertainties of about 10 km s^-1. We combine our sample of 30 radial velocities for globular clusters around NGC 3379 with 8 additional GC velocities from the literature, and find a projected velocity dispersion of sig<sub>p</sub>

= 175⁺²⁴_-22 km s^-1 at R < 5' and sig<sub>p</sub>

= 147⁺⁴⁴_-39 at R > 5'. These velocity dispersions are consistent with a dark matter halo around NGC 3379 with a concentration in the range expected from a LCDM cosmological model and a total mass of ~6x10¹¹ M_Sun. Such a model is also a consistent with the stellar velocity dispersion at small radii and the rotation of the HI ring at large radii, and has a (M/L)_B that increases by a factor of five from several kpc to 100 kpc. Our velocity dispersion for the globular cluster system of NGC 3379 is somewhat higher than that found for the planetary nebulae (PNe) in the inner region covered by the PN data, and we discuss possible reasons for this difference. For NGC 3384, we find the GC system has a rotation signature broadly similar to that seen in other kinematic probes of this SB0 galaxy. This suggests that significant rotation may not be unusual in the GC systems of disc galaxies.

Mass-to-light ratio gradients in early-type galaxy haloes
N. R. Napolitano, M. Capaccioli, A. J. Romanowsky, N. G. Douglas, M. R. Merrifield, K. Kuijken, M. Arnaboldi, O. Gerhard, K. C. Freeman
Monthly Notices of the Royal Astronomical Society, Volume 357, Issue 2, February 2005, pp. 691-706
MNRAS (also here) | ADS | astro-ph/0411639
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51]

Owing to the fact that the near future should see a rapidly expanding set of probes of the halo masses of individual early-type galaxies, we introduce a convenient parameter for characterising the halo masses from both observational and theoretical results: \dML, the logarithmic radial gradient of the mass-to-light ratio. Using halo density profiles from Lambda-cold dark matter (CDM) simulations, we derive predictions for this gradient for various galaxy luminosities and star formation efficiencies e_SF. As a pilot study, we assemble the available \dML\ data from kinematics in early-type galaxies -- representing the first unbiased study of halo masses in a wide range of early-type galaxy luminosities -- and find a correlation between luminosity and \dML, such that the brightest galaxies appear the most dark-matter dominated. We find that the gradients in most of the brightest galaxies may fit in well with the LCDM predictions, but that there is also a population of fainter galaxies whose gradients are so low as to imply an unreasonably high star formation efficiency e_SF > 1. This difficulty is eased if the dark haloes are not assumed to have the standard LCDM profiles, but instead have lower central concentrations.

Tracing the star stream through M31 using planetary nebula kinematics
H. R. Merrett, K. Kuijken, M. R. Merrifield, A. J. Romanowsky, N. G. Douglas, N. R. Napolitano, M. Arnaboldi, M. Capaccioli, K. C. Freeman, O. Gerhard, N. W. Evans, M. I. Wilkinson, C. Halliday, T. J. Bridges, D. Carter
Monthly Notices of the Royal Astronomical Society, Volume 346, Issue 4, December 2003, pp. L62-L66
MNRAS (also here) | ADS | astro-ph/0311090 | local PDF | local gzipped postscript
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33]

We present a possible orbit for the Southern Stream of stars in M31, which connects it to the Northern Spur. Support for this model comes from the dynamics of planetary nebulae (PNe) in the disk of M31: analysis of a new sample of 2611 PNe obtained using the Planetary Nebula Spectrograph reveals ~20 objects whose kinematics are inconsistent with the normal components of the galaxy, but which lie at the right positions and velocities to connect the two photometric features via this orbit. The satellite galaxy M32 is coincident with the stream both in position and velocity, adding weight to the hypothesis that the stream comprises its tidal debris.

A dearth of dark matter in ordinary elliptical galaxies
A. J. Romanowsky, N. G. Douglas, M. Arnaboldi, K. Kuijken, M. R. Merrifield, N. R. Napolitano, M. Capaccioli, & K. C. Freeman
Science, Volume 301, Issue 5640, 19 September 2003, pp. 1696-1698
free Science Online text | ADS | astro-ph/0308518 | free Science Online abstract | Science Express | local PDF | local gzipped postscript
Planetary Nebula Spectrograph homepage
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84] [85] [86] [87] [88] [89] [90] [91] [92] [93] [94] [95] [96] [97] [98] [99] [100] [101] [102] [103] [104] [105] [106] [107] [108] [109] [110] [111] [112] [113] [114] [115] [116] [117] [118] [119] [120] [121] [122] [123] [124] [125] [126] [127] [128] [129] [130] [131] [132] [133] [134] [135] [136] [137] [138] [139] [140] [141] [142] [143] [144] [145] [145] [146] [147] [148] [149] [150] [151] [152] [153] [154] [155] [156] [157] [158] [159] [160] [161] [162] [163] [164] [165] [166] [167] [168] [169] [170] [171] [172] [173] [174] [175] [176] [177] [178] [179] [180] [181] [182] [183] [184] [185] [186] [187] [188] [189] [190] [191] [192] [193] [194] [195] [196] [197] [198] [199] [200] [201] [202] [203]

The kinematics of the outer parts of three intermediate-luminosity elliptical galaxies were studied with the Planetary Nebula Spectrograph. The galaxies' velocity dispersion profiles were found to decline with the radius, and dynamical modeling of the data indicates the presence of little if any dark matter in these galaxies' halos. This unexpected result conflicts with findings in other galaxy types and poses a challenge to current galaxy formation theories.

The Planetary Nebula Spectrograph: the green light for galaxy kinematics
N. G. Douglas, M. Arnaboldi, K. C. Freeman, K. Kuijken, M. R. Merrifield, A. J. Romanowsky, K. Taylor, M. Capaccioli, T. Axelrod, R. Gilmozzi, J. Hart, G. Bloxham, & D. Jones
Publications of the Astronomical Society of the Pacific, Volume 114, Number 801, November 2002, pp. 1234-1251
PASP | ADS | astro-ph/0208149 | local compressed postscript?!
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32]
PN.S homepage

Planetary nebulae (PNe) are now well established as probes of galaxy dynamics and as standard candles in distance determinations. Motivated by the need to improve the efficiency of planetary nebulae searches and the speed with which their radial velocities are determined, a dedicated instrument - the Planetary Nebulae Spectrograph or PN.S - has been designed and commissioned at the 4.2m William Herschel Telescope. The high optical efficiency of the spectrograph results in the detection of typically ~150 PNe in galaxies at the distance of the Virgo cluster in one night of observations. In the same observation the radial velocities are obtained with an accuracy of ~20 km s^-1.

Dynamics of stars and globular clusters in M87
Aaron J. Romanowsky & Christopher S. Kochanek
The Astrophysical Journal, Volume 553, Number 2, 1 June 2001, pp. 722-732
ApJ | ADS | astro-ph/0008062 | local gzipped postscript?!
Image of galaxy #1 | Image of galaxy #2 | Image of galaxy #3 | Radio images | Image of disk | Virgo cluster in X-rays | Hubble Heritage page
Surface brightness | Velocity profiles | Globular clusters | more info
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55[ [56] [57] [58] [59] [60] [61]

We examine the dynamics of the stars and globular clusters in the nearby giant galaxy M87 and constrain the mass distribution, using all the available data over a large range of radii, including higher-order moments of the stellar line-of-sight velocity distributions and the discrete velocities of over two hundred globular clusters. We introduce an extension of spherical orbit modeling methods that makes full use of all the information in the data, and provides very robust constraints on the mass models. We conclusively rule out a constant mass-to-light ratio model, and infer that the radial density profile of the galaxy's dark halo falls off more slowly than r^-2, suggesting that the potential of the Virgo Cluster is already dominant at r ~ 300" ~ 20 kpc.

Constraints on H₀ from the central velocity dispersions of lens galaxies
Aaron J. Romanowsky & Christopher S. Kochanek
The Astrophysical Journal, Volume 516, Number 1, 1 May 1999, pp. 18-26
ApJ | ADS | astro-ph/9805080 | local gzipped postscript?!
Q0957+561 data and images | PG 1115+080 data, images, info: [1] [2] [3]
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49]

We employ Schwarzschild's method of orbit modeling to constrain the mass profiles of the central lens galaxies in Q0957+561 and PG 1115+080. We combine the measured central projected stellar velocity dispersions of these galaxies with the self-similar radial profiles of the rms velocity and of the Gauss-Hermite moment h₄ observed in nearby galaxies for 0 &lsim

2 R_eff. For Q0957+561, we find a 16% uncertainty in the galaxy mass, and formal 2-sigma limits on the Hubble constant of H₀ = 61_-15⁺¹³ km s^-1 Mpc^-1. For PG 1115+080, we find that none of the viable lens models can be ruled out, so that H₀ is not yet strongly constrained by this system.

Twisting of X-ray isophotes in triaxial galaxies
Aaron J. Romanowsky & Christopher S. Kochanek
The Astrophysical Journal, Volume 493, Number 2, 1 February 1998, pp. 641-649
ApJ | ADS | astro-ph/9708212 | gzipped postscript?!
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]
NGC 720 update (also here)

We investigate X-ray isophote twists created by triaxiality differences between the luminous stellar distributions and the dark halos in elliptical galaxies. For a typically oblate luminous galaxy embedded in a more prolate halo formed by dissipationless collapse, the triaxiality difference of &Delta

0.7 leads to typical isophote twists of &langle

16° ± 19° at 3 stellar effective radii. In a model which includes baryonic dissipation the effect is smaller, with &Delta

0.3 and

5° ± 8°. Thus, accurate measurements of X-ray isophote twists may be able to set constraints on the interactions between baryons and dissipationless dark matter during galaxy formation. The 30° X-ray isophote twist in the E4 galaxy NGC 720 cannot be reproduced by our model, suggesting an intrinsic misalignment between the halo and the stars rather than a projection effect.

Structural and dynamical uncertainties in modelling axisymmetric elliptical galaxies
Aaron J. Romanowsky & Christopher S. Kochanek
Monthly Notices of the Royal Astronomical Society, Volume 287, Issue 1, 1 May 1997, pp. 35-50
ADS | astro-ph/9609202 | local gzipped postscript?!
NGC 7619 images
Citations: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]

Quantitative dynamical models of galaxies require deprojection of the observed surface brightness to determine the luminosity density of the galaxy. Existing deprojection methods for axisymmetric galaxies assume that a unique deprojection exists for any given inclination, even though the projected density is known to be degenerate to the addition of 'konus densities' that are invisible in projection. We develop a deprojection method based on linear regularization that can explore the range of luminosity densities that are statistically consistent with an observed surface brightness distribution. The luminosity density is poorly constrained at modest inclinations (i &gsim

30° ), even in the limit of vanishing observational errors. In constant mass-to-light ratio, axisymmetric, two-integral dynamical models, the uncertainties in the luminosity density result in large uncertainties in the meridional plane velocities. However, the projected line-of-sight velocities show variations comparable to current typical observational uncertainties.

Unrefereed Papers

Structure and substructure of galactic spheroids
A. J. Romanowsky, J. P. Brodie, J. S. Bullock, R. Ciardullo, P. Guhathakurta, L. Hoffman, K. A. G. Olsen, J. R. Primack, G. van den Ven
Astro2010 Science White Paper, arXiv:0902.3025
ADS | arXiv:0902.3025

The full spatio-chemo-dynamical structure of galaxies of all types and environments at low redshift provides a critical accompaniment to observations of galaxy formation at high redshift. The next decade brings the observational opportunity to strongly constrain nearby galaxies' histories of star formation and assembly, especially in the spheroids that comprise the large majority of the stellar mass in the Universe but have until now been difficult to study. In order to constrain the pathways to building up the spheroidal "red-sequence", various standard techniques in photometry and spectroscopy, particularly with resolved tracer populations like globular clusters and planetary nebulae, can be scaled up to comprehensive surveys as improved wide-field instrumentation is increasingly available. At the same time, progress in adaptive optics on giant telescopes could for the first time permit deep, resolved photometric and spectroscopic analysis of large samples of individual stars in these systems, thereby revolutionizing galaxy studies. Strong theoretical support is needed in order to understand the new observational constraints via detailed modeling and self-consistent simulations of star and galaxy formation throughout cosmic time.

The star formation histories of disk and E/S0 galaxies from resolved stars
K. A. G. Olsen, A. J. Romanowsky, A. Saha, E. Skillman, B. F. Williams, R. F. G. Wyse
Astro2010 Science White Paper, arXiv:0902.4216
ADS | arXiv:0902.4216

The resolved stellar populations of local galaxies, from which it is possible to derive complete star formation and chemical enrichment histories, provide an important way to study galaxy formation and evolution that is complementary to lookback time studies. We propose to use photometry of resolved stars to measure the star formation histories in a statistical sample of galaxy disks and E/S0 galaxies near their effective radii. These measurements would yield strong evidence to support critical questions regarding the formation of galactic disks and spheroids. The main technological limitation is spatial resolution for photometry in heavily crowded fields, for which we need improvement by a factor of ~10 over what is possible today with filled aperture telescopes.

The Planetary Nebula Spectrograph successfully commissioned
M. R. Merrifield, N. G. Douglas, K. Kuijken, and A. J. Romanowsky, for the PN.S Consortium
The ING Newsletter, Number 5, October 2001, pp. 17-18
ING | ADS
Citations: [1]

Extragalactic planetary nebula kinematics with the WHT
Aaron J. Romanowsky, Nigel G. Douglas, Magda Arnaboldi, and Konrad Kuijken
The ING Newsletter, Number 4, March 2001, pp. 23-25
ING | ADS | KAI preprint?!
Citations: [1]

The use of planetary nebulae (PNe) as tracer particles is a promising approach to studying the kinematics of the outskirts of nearby elliptical galaxies. We report preliminary results from observations and modelling in the giant elliptical NGC 4472 (= M49), using the AUTOFIB2/WYFFOS multifibre spectrograph at the 4.2-m William Herschel Telescope. We also introduce the Planetary Nebula Spectrograph, a new instrument expressly designed for measuring the kinematics of PNe.

Conference Proceedings

Digging for formational clues in the halos of early-type galaxies
A.J. Romanowsky
in Hunting for the Dark: The Hidden Side of Galaxy Formation, ed. V.P. Debattista & C.C. Popescu, in press, arXiv:1001.3138
ADS
Citations: [1] [2]

Many of the fundamental properties of early-type galaxies (ellipticals and lenticulars) can only be accessed by venturing beyond their oft-studied centers into their large-radius halo regions. Advances in observations of kinematical tracers allow early-type halos to be increasingly well probed. This review focuses on recent findings on angular momentum and dark matter content, and discusses some possible implications for galaxy structure and formation.

The orbital structure of the massive elliptical galaxy, NGC 5846
P. Das, O. Gerhard, L. Coccato, E. Churazov, W. Forman, A. Finoguenov, H. Böhringer, M. Arnaboldi, M. Capaccioli, A. Cortesi, F. de Lorenzi, N.G. Douglas, K.C. Freeman, K.C. K. Kuijken, M.R. Merrifield, N.R. Napolitano, E. Noordermeer, A.J. Romanowsky
Astronomische Nachrichten, 2008, Vol. 329, pp. 940-943
ADS

We use density and temperature profiles obtained from XMM-Newton observations to derive a potential of NGC 5846 out to 11 R_e, thus probing the mass distribution deep into the halo. The inferred circular velocity is significantly higher than the extrapolation of dynamical models implying a halo, more massive than previously thought. Using an I-band surface-brightness profile and a projected velocity dispersion profile consisting of long-slit kinematic measurements and planetary nebulae (PNe) velocity dispersions, we solve the Jeans equations, assuming a non-rotating spherical system. The solutions suggest a highly radially anisotropic galaxy outside 0.7 R_e with beta ~ 0.75.

Probing the early-type galaxy halos using planetary nebulae as kinematic tracers
L. Coccato, O. Gerhard, M. Arnaboldi, P. Das, N.G. Douglas, K. Kuijken, M.R. Merrifield, N.R. Napolitano, E. Noordermeer, A.J. Romanowsky, M. Capaccioli, A. Cortesi, F. De Lorenzi, K.C. Freeman
Astronomische Nachrichten, 2008, Vol. 329, p. 912

We present first results of a study of the halo kinematics for a sample of early type galaxies using planetary nebulae (PNe) as kinematical tracers. PNe allow to extend up to several effective radii (R_e) the information from absorption line kinematics (confined to within 1 or 2 R_e), providing valuable information and constraints for merger simulations and galaxy formation models. We find that the specific angular momentum per unit mass has a more complex radial dependence when the halo region is taken into account and that the halo velocity dispersion is related to the total galaxy luminosity, isophotal shape, and number of PNe per unit of luminosity.

Dark-matter content of early-type galaxies with planetary nebulae
N.R. Napolitano, A.J. Romanowsky, L. Coccato, M. Capaccioli, N.G. Douglas, E. Noordermeer, M.R. Merrifield, K. Kuijken, M. Arnaboldi, O. Gerhard, K.C. Freeman, F. De Lorenzi, P. Das
Dark Galaxies & Lost Baryons, Proceedings of the International Astronomical Union, IAU Symposium, Volume 244, 2008, ed. J.I. Davies & M.J. Disney (Cambridge: Cambridge Univ. Press), pp. 289-294
ADS | arXiv:0709.1636
Citations: [1] [2] [3]

We examine the dark matter properties of nearby early-type galaxies using planetary nebulae (PNe) as mass probes. We have designed a specialised instrument, the Planetary Nebula Spectrograph (PN.S) operating at the William Herschel telescope, with the purpose of measuring PN velocities with best efficiency. The primary scientific objective of this custom-built instrument is the study of the PN kinematics in 12 ordinary round galaxies. Preliminary results showing a dearth of dark matter in ordinary galaxies (Romanowsky et al. 2003) are now confirmed by the first complete PN.S datasets. On the other hand early-type galaxies with a "regular" dark matter content are starting to be observed among the brighter PN.S target sample, thus confirming a correlation between the global dark-to-luminous mass virial ratio (f_DM=M_DM/M_star) and the galaxy luminosity and mass.

Kinematics of globular cluster systems
A. J. Romanowsky
Globular Clusters - Guides to Galaxies, 2009, ed. T. Richtler & S. Larsen (Berlin: Springer), pp. 433-443
ADS1 | ADS2 | astro-ph/0609251
Citations: [1] [2] [3] [4] [5] [6] [7]

I review the field of globular cluster system (GCS) kinematics, including a brief primer on observational methods. The kinematical structures of spiral galaxy GCSs so far appear to be broadly similar. The inferred rotation and mass profiles of elliptical galaxy halos exhibit a diversity of behaviors, requiring more systematic observational and theoretical studies.

Planetary nebulae as mass tracers in galaxies
A. J. Romanowsky
Planetary Nebulae in Our Galaxy and Beyond, Proceedings of the International Astronomical Union, Symposium #234, 2006, ed. M.J. Barlow & R.H. Mendez (Cambridge: Cambridge Univ. Press), pp. 341-348
(invited review talk at Planetary Nebulae in our Galaxy and beyond in April 2006)
ADS | astro-ph/0605275
Citations: [1] [2]

Planetary nebula are useful kinematic tracers of the stars in all galaxy types. I review recent observationally-driven developments in the study of galaxy mass profiles. These have yielded surprising results on spiral galaxy disk masses and elliptical galaxy halo masses. A key remaining question is the coupling between PNe and the underlying stellar populations.

Probing the environment with galaxy dynamics
A. J. Romanowsky
Groups of Galaxies in the Nearby Universe, ESO Astrophysics Symposia, 2007, ed. I. Saviane, V.D. Ivanov & J. Borissova (Berlin: Springer), pp. 385-389
(presented at Groups of Galaxies in the Nearby Universe in Dec 2005)
ADS | astro-ph/0605057

I present various projects to study the halo dynamics of elliptical galaxies. This allows one to study the outer mass and orbital distributions of ellipticals in different environments, and the inner distributions of groups and clusters themselves.

Dynamics in galaxy halos
A. J. Romanowsky
XI IAU Regional Latin American Meeting of Astronomy, 2006, ed.. L. Infante & M. Rubio, RevMexAA SC, Volume 26, p. 198
(presented at 11th Annual Latin-American Regional IAU Meeting)
ADS

The outskirts of galaxies offer crucial clues about the formation history. There are clear predictions in the LCDM paradigm for the distribution of mass, angular momentum, and orbit types in galaxy halos. I present work from several interrelated programs to study the dynamics of nearby galaxy halos -- including a systematic survey of ordinary elliptical galaxies. The observational probes include X-ray emission and the kinematics of stars, planetary nebulae, and globular clusters, Initial results imply dark matter halos in L* galaxies with concentrations too low for LCDM, while in brighter galaxies, there appears to be too much dark matter for the MOND gravitational theory.

NGC 1399 and MOND
T. Richtler, Y. Schuberth, & A. J. Romanowsky
XI IAU Regional Latin American Meeting of Astronomy, 2006, ed.. L. Infante & M. Rubio, RevMexAA SC, Volume 26, p. 198
(presented at 11th Annual Latin-American Regional IAU Meeting)
ADS
Citations: [1]

Modified Newtonian Dynamics (MOND), having its succeses in the phenomenology of rotation curves of disk galaxies, is known to not remove the need for dark matter in galaxy clusters. Does this also apply to central elliptical galaxies? Our sample of globular cluster velocities in NGC 1399, the centra galaxy of the Fornax cluster, now comprises 625 objects out to a galactocentric distance of 100 kpc, extending the sample of Richtler et al. (2004). In NGC 1399, the deep MOND regimeis realized only at radial distances larger than 100 kpc and one has to apply partly physically unmotivated interpolations between the Newtonian and the MONDian regime. For any of the proposed interpolation schemes, MOND is not able to explain the circular velocity indicated by the globular cluster analysis, so dark matter is still needed. Bekenstein's (2004) interpolation, if applied over the full radial range, might be considered as being consistent with no dark matter. However, preliminary analyses suggest that Bekenstein's interpolations fails to remove the need for dark matter in other central galaxies like NGC 3311 or NGC 6166, so the case of NGC 1399 probably cannot be generalized. Moreover, it predicts too high circular velocities for NGC 4636.

Elliptical galaxy halo masses from internal kinematics
A. J. Romanowsky
Mass Profiles and Shapes of Cosmological Structures, EAS Publications Series, Vol. 20, 2006, ed. G. A. Mamon, F. Combes, C. Deffayet, & B. Fort (Paris: EDP Sciences), pp. 119-126
(invited review talk at XXIst IAP Colloquium in Jul 2005)
ADS | EAS | astro-ph/0510183
Citations: [1] [2] [3] [4] [5]

The halo masses of nearby individual elliptical galaxies can be estimated by using the kinematics of their stars, planetary nebulae, and globular clusters -- ideally in combination. With currently improving coverage of galaxies of ordinary luminosities and morphologies, systematic trends may be identified. Bright, boxy ellipticals show strong signatures of dark matter, while faint, disky ones typically do not. The former result is problematic for the MOND theory of gravity, and the latter is a challenge to explain in the LCDM paradigm of galaxy formation.

Planetary nebulae as dynamical tracers: mass-to-light-ratio gradients in early-type galaxies
N.R. Napolitano, A.J. Romanowsky, M. Capaccioli, K. Kuijken, M.R. Merrifield, N.G. Douglas, M. Arnaboldi, K.C. Freeman & O. Gerhard
Planetary Nebulae beyond the Milky Way, ESO Astrophysics Symposia, 2006, ed. L. Stanghellini, J.R. Walsh & N.G Douglas (Berlin: Springer), pp. 324-328
(presented at Planetary Nebulae Beyond the Milky Way in May 2004)
ADS

Planetary Nebulae (PNe) have enabled mass-to-light ratios (M/L) in early-type galaxies to be constrained to unprecedented distance from the center, showing in some cases clear evidence of increasing M/L, in other caes fairly constant M/L. We combine the information obtained from PN kinematics with radially extended long-slit spectroscopy data in order to constrain the M/L trends in a heterogeneous sample of early-type galaxies. We discuss whether these trends are expected in the LCDM framework of galaxy formation.

Probing halos with PNe: mass and angular momentum in early-type galaxies
A. J. Romanowsky
Planetary Nebulae beyond the Milky Way, ESO Astrophysics Symposia, 2006, ed. L. Stanghellini, J.R. Walsh & N.G Douglas (Berlin: Springer), pp. 294-298
(presented at Planetary Nebulae Beyond the Milky Way in May 2004)
ADS | astro-ph/0411797
Citations: [1]

We present an observational survey program using planetary nebulae, globular clusters, and X-ray emission to probe the halos of early-type galaxies. We review evidence for scanty dark matter halos around ordinary elliptical galaxies, and discuss the possible implications. We also present measurements of rotation in the halos.

Mapping the stellar dynamics of M31
M. Merrett, M. Merrifield, K. Kuijken, A. Romanowsky, N. Douglas, N. Napolitano, M. Arnaboldi, M. Capaccioli, K. Freeman, O. Gerhard, D. Carter, N. W. Evans, M. Wilkinson, C. Halliday, T. Bridges
Planetary Nebulae beyond the Milky Way, ESO Astrophysics Symposia, 2006, ed. L. Stanghellini, J.R. Walsh & N.G Douglas (Berlin: Springer), pp. 281-285
(presented at Planetary Nebulae Beyond the Milky Way in May 2004)
ADS | astro-ph/0407331
Citations: [1] [2] [3]

Using the Planetary Nebula Spectrograph, we have observed and measured the velocities for some 2764 PNe in the disk and halo of the Andromeda galaxy. Preliminary analysis using a basic ring model shows a rotation curve in good agreement with that obtained from HI data out to ~20kpc. Some substructure has also been detected within the velocity field, which can be modeled as the continuation of the tidal-remnant known as the Southern Stream, as it passes through Andromeda's disk.

Halo masses of early-type galaxies: theory vs observation
A. J. Romanowsky, N. R. Napolitano, M. Capaccioli, N. G. Douglas, M. R. Merrifield, K. Kuijken, M. Arnaboldi, O. Gerhard, & K. C. Freeman
Bulletin of the American Astronomical Society, Volume 36, Number 5, 2004, p. 1397 (31.07)
(presented at the 205th meeting of the American Astronomical Society in Jan 2005)
ADS

We introduce a mass-to-light gradient parameter to describe the amount of dark matter surrounding early-type galaxies. Assembling kinematic data from the literature on the halo masses of 20 galaxies, we find a correlation between luminosity and gradient. The gradients do not fit in well with predictions from LCDM, requiring either baryon excess or diffuse halos
We also describe various programs using planetary nebulae, globular clusters, and X-rays to jointly determine galaxy halo masses.

Some dynamical constraints on the halo of NGC 3379 from wide-field spectroscopy of its globular cluster system
S. E. Zepf, G. Bergond, A. J. Romanowsky, K. L. Rhode, & R. M. Sharples
Bulletin of the American Astronomical Society, Volume 36, Number 5, 2004, p. 1397 (31.04)
(presented at the 205th meeting of the American Astronomical Society in Jan 2005)
ADS
Citations: [1] [2]

We present initial results from a study of the dynamics of the outer halo of the nearby elliptical galaxy NGC 3379 based on wide-field spectroscopy of its globular cluster system. Our study uses the FLAMES multi-object spectrograph at the VLT to determine the radial velocities of candidate gloublar clusters previously identified in CCD Mosaic images of nearby elliptical galaxies. Over the large radial range of 0.7 to 15 arcminutes (2 to 45 kpc) covered by our study, the velocity dispersion of the globular cluster system suggests the presence of a dark matter halo around NGC 3379, consistent with the observed rotation of an HI ring at larger distances. A less significant dark matter halo has been suggested by studies of the planetary nebulae, and we discuss possible reasons for this difference.

Galaxy dynamics with the Planetary Nebula Spectrograph
N. R. Napolitano, A. J. Romanowsky, N. G. Douglas, M. Capaccioli, M. Arnaboldi, K. Kuijken, M. R. Merrifield, K. C. Freeman, O. Gerhard
Memorie della Societa Astronomica Italiana Supplement, Volume 5, 2004, pp. 255-260
ADS | astro-ph/0406419
Citations: [1]

The Planetary Nebula Spectrograph is a dedicated instrument for measuring radial velocity of individual Planetary Nebulae (PNe) in galaxies. This new instrument is providing crucial data with which to probe the structure of dark halos in the outskirts of elliptical galaxies in particular, which are traditionally lacking of easy interpretable kinematical tracers at large distance from the center. Preliminary results on a sample of intermediate luminosity galaxies have shown little dark matter within ~5 R_eff implying halos either not as massive or not as centrally concentrated as CDM predicts (Romanowsky et al. 2003). We briefly discuss whether this is consistent with a systematic trend of the dark matter content with the luminosity as observed in an extended sample of early-type galaxies.

Halo tracers in nearby galaxies
A. J. Romanowsky, N. G. Douglas, K. Kuijken, M. R. Merrifield, M. Arnaboldi, H. Merrett, N. R. Napolitano, M. Capaccioli, K. C. Freeman, G. Bergond, R. M. Sharples, S. E. Zepf, K. L. Rhode
Bulletin of the American Astronomical Society, Volume 35, Number 5, 2003, p. 1312 (65.05)
(presented at the 203rd meeting of the American Astronomical Society in Jan 2004)
ADS

We present results from various programs studying the halo kinematics of nearby galaxies. These include more extensive planetary nebula (PN) data and updated mass determinations of the "ordinary ellipticals" recently found to show little trace of dark matter. Our globular cluster study of NGC 3379 independently confirms the low halo mass of this galaxy. PN measurements from a larger galaxy sample show weaker rotation than expected from galaxy mergers. We also present 2800 PN velocities around M31, and from these data infer a dynamical connection between the halo stellar streamer, the northern spur, and M32.

Elliptical galaxies: darkly cloaked or scantily clad?
A. J. Romanowsky, N. G. Douglas, K. Kuijken, M. R. Merrifield, M. Arnaboldi, N. R. Napolitano, H. Merrett, M. Capaccioli, K. C. Freeman, O. Gerhard.
Dark Matter in Galaxies, IAU Symposium Vol. 220, 2004, ed. S. Ryder, D. J. Pisano, M. Walker, & K. Freeman (San Francisco: ASP), pp. 165-170
(presented at Dark Matter in Galaxies in July 2003)
ADS | astro-ph/0310874 | local PDF | local gzipped postscript
Citations: [1] [2] [3] [4] [5]

Planetary nebulae (PNe) may be the most promising tracers in the halos of early-type galaxies. We have used multi-object spectrographs on the WHT and the VLT, and the new Planetary Nebula Spectrograph on the WHT, to obtain hundreds of PN velocities in a small sample of nearby galaxies. These ellipticals show weak halo rotation, which may be consistent with ab initio models of galaxy formation, but not with more detailed major merger simulations. The galaxies near L^* show evidence of a universal declining velocity dispersion profile, and dynamical models indicate the presence of little dark matter within 5 R_eff -- implying halos either not as massive or not as centrally concentrated as CDM predicts.

Early-type galaxy halo dynamics inferred using the PN Spectrograph
N. G. Douglas, A. J. Romanowsky, K. Kuijken, M. R. Merrifield, N. R. Napolitano, M. Arnaboldi, K. C. Freeman, M. Capaccioli, O. Gerhard
Dark Matter in Galaxies, IAU Symposium Vol. 220, 2004, ed. S. Ryder, D. J. Pisano, M. Walker, & K. Freeman (San Francisco: ASP), pp. 171-172
(presented at Dark Matter in Galaxies in July 2003)
ADS | astro-ph/0310900

A new instrument is providing crucial data with which to probe the structure of dark halos in elliptical galaxies.

Is there a dichotomy in the dark matter as well as in the baryonic matter properties of ellipticals?
N. R. Napolitano, M. Capaccioli, M. Arnaboldi, M. R. Merrifield, N. G. Douglas, K. Kuijken, A. J. Romanowsky, K. C. Freeman
Dark Matter in Galaxies, IAU Symposium Vol. 220, 2004, ed. S. Ryder, D. J. Pisano, M. Walker, & K. Freeman (San Francisco: ASP), pp. 173-174
(presented at Dark Matter in Galaxies in July 2003)
ADS | astro-ph/0310798
Citations: [1] [2]

We have found a correlation between the M/L global gradients and the structural parameters of the luminous components of a sample of 19 early-type galaxies. Such a correlation supports the hypothesis that there is a connection between the dark matter content and the evolution of the baryonic component in such systems.

Modern techniques in galaxy kinematics: results from planetary nebula spectroscopy
Aaron J. Romanowsky, Nigel G. Douglas, Konrad Kuijken, Magda Arnaboldi, Joris Gerssen, & Michael R. Merrifield
Planetary Nebulae: Their Evolution and Role in the Universe, IAU Symposium Vol. 209, 2003, ed. S. Kwok, M. Dopita, & R. Sutherland (San Francisco: ASP), pp. 639-640
(presented at Planetary Nebulae: Their Evolution and Role in the Universe in Nov 2001)
ADS | astro-ph/0208118 | local postscript

We have observed planetary nebulae (PNe) in several early-type galaxies using new techniques on 4- to 8-meter-class telescopes. We obtain the first large data sets ( &gsim

100 velocities each) of PN kinematics in galaxies at &gsim

15 Mpc, and present some preliminary dynamical results.

Modern techniques in galaxy kinematics: CDI and the Planetary Nebula Spectrograph
N. G. Douglas, K. Kuijken, A. J. Romanowsky, M. R. Merrifield, M. Arnaboldi, K. Freeman, & K. Taylor
Planetary Nebulae: Their Evolution and Role in the Universe, IAU Symposium Vol. 209, 2003, ed. S. Kwok, M. Dopita, & R. Sutherland (San Francisco: ASP), pp. 637-638
(presented at Planetary Nebulae: Their Evolution and Role in the Universe in Nov 2001)
ADS | astro-ph/0207556 | local postscript

We report here the successful commissioning of the PN.Spectrograph, the first special-purpose instrument for the measurement of galaxy kinematics through the PN population.

Dynamics of stars and globular clusters in galaxy halos
Aaron J. Romanowsky, Nigel G. Douglas, Konrad Kuijken, Magda Arnaboldi, Markus Kissler-Patig, Ray M. Sharples, Stephen E. Zepf, & Katherine L. Rhode
Extragalactic Globular Cluster Systems, ESO Astrophysics Symposia, 2003, ed. M. Kissler-Patig (Berlin: Springer), pp. 310-313
(presented at Extragalactic Globular Cluster Systems in Aug 2002)
ADS | local gzipped postscript
Citations: [1]

We have obtained kinematical data in the halos of the giant ellipticals M49 and M87. These include globular cluster velocities in M49 to 10 R_eff and planetary nebula velocities in M49 and M87 to 4 R_eff. We report initial results, including dynamical comparisons between the diffuse stellar components and globular cluster systems.

Dynamical constraints on early-type galaxy halos
Aaron J. Romanowsky, Magda Arnaboldi, Nigel G. Douglas, Konrad Kuijken, Michael R. Merrifield, Kenneth C. Freeman, & Joris Gerssen
Galaxy Evolution: Theory and Observations, 2003, ed. V. Avila-Reese, C. Firmani, C. S. Frenk, & C. Allen, RevMexAA SC, Volume 17, August 2003, p. 45
(presented at Galaxy Evolution: Theory and Observations in Apr 2002)
ADS (also here) | local PDF | local compressed postscript
Citations: [1]

We report results from several ongoing programs to probe the halo dynamics of nearby elliptical and S0 galaxies by measuring the radial velocities of planetary nebulae (PNe) in their outer parts. These include observations with ISIS and WYFFOS/AUTOFIB2 on the WHT, FORS2/MXU on UT2, and the newly commissioned Planetary Nebula Spectrograph on the WHT. We have obtained velocities of 70-200 PNe in each of 7 galaxies, which we are using in combination with integrated-light stellar kinematics, globular cluster kinematics, and X-ray emission to determine the distribution of mass, orbit structure, and angular momentum in the galaxy halos, for comparison to predictions from galaxy formation models.

Mass distributions in early-type galaxy halos
Aaron J. Romanowsky, Nigel G. Douglas, Konrad Kuijken, Michael R. Merrifield, Magda Arnaboldi, Kenneth C. Freeman, & Keith Taylor
The Mass of Galaxies at Low and High Redshift, ESO Astrophysics Symposia, 2003, ed. R. Bender & A. Renzini (Berlin: Springer), pp. 72-73
(presented at The Mass of Galaxies at Low and High Redshift in Oct 2001)
ADS | local PDF | local gzipped postscript

One of the most promising avenues for determining the distribution of mass in the outer parts of early-type galaxies is through the kinematics of planetary nebulae (PNe). We have used new techniques and instrumentation on the WHT and the VLT to obtain velocities for hundreds of PNe around several nearby galaxies. We show simple mass models and describe more rigorous orbit modeling methods for the combined analysis of different dynamical constraints in galaxy halos.

Dynamics of globular cluster systems in elliptical galaxies
Aaron J. Romanowsky
Extragalactic Star Clusters, IAU Symposium Vol. 207, 2002, ed. D. Geisler, E. K. Grebel, & D. Minniti (San Francisco: ASP), pp. 336-339
(presented at Extragalactic Star Clusters in March 2001)
ADS | local gzipped postscript

One of the most promising avenues for exploring the dynamics of the outer parts of elliptical galaxies involves using bright discrete objects as kinematical tracers: globular clusters and planetary nebulae. As large data sets are becoming available, rigorous dynamical analyses are needed to interpret them. To this end, we have developed an extension of orbit modeling methods, which we apply to the dynamics of the giant ellipticals M87 and M49. We find that a sample of ~200 globular clusters velocities is effective at demonstrating the presence of a dark halo and providing hints about the detailed distribution of the dark matter.

Tracing early-type galaxy halo dynamics
A. J. Romanowsky, N. G. Douglas, K. Kuijken, M. R. Merrifield, M. Arnaboldi, K. C. Freeman, K. Taylor
Bulletin of the American Astronomical Society, Volume 33, Number 4, 2001, p. 1533 (153.03)
(presented at the 199th meeting of the American Astronomical Society in Jan 2002)
ADS
Citations: [1]

Knowledge about the dynamical properties of the halos of early-type galaxies (S0's and ellipticals) has been slow to progress because of the dearth of suitable tracers. To address this shortcoming, we have embarked upon a program to obtain kinematics of planetary nebulae (PNe) around nearby galaxies, using general-purpose multi-object spectrographs on 4-8m class telescopes, and using the newly commissioned Planetary Nebula Spectrograph, a specialized binocular instrument for counter-dispersed imaging. We present measurements of ~100-200 PN velocities to large radii around the galaxies NGC 821, NGC 4472, NGC 4486, and NGC 7457. From these, we derive the galaxies' mass distributions and stellar angular momenta. We compare these to predictions from cosmological simulations, and to results from globular cluster system kinematics and X-ray halo constraints.

Kinematics of planetary nebulae in NGC 4472
Aaron J. Romanowsky, Nigel G. Douglas, Konrad Kuijken, Magda Arnaboldi
Bulletin of the American Astronomical Society, Volume 33, Number 1, 2001, p. 719 (134.01)
(presented at the 197th meeting of the American Astronomical Society in January 2001)
ADS

Unlike the case of spiral galaxies, the mass distribution in the outer parts of elliptical galaxies is in general poorly known, owing to a paucity of suitable observational tracers. One effective observational approach exploits the kinematics of planetary nebulae (PNe). With much of their emission concentrated in the [O III] line at 5007Å, these objects can be readily identified against the background light to large radii, and their velocities easily measured. Since the PNe are representative of the general stellar population of the galaxy, measurements of their velocities can be used to infer the dynamics and mass distribution of the galaxy as a whole.

Using PNe around the giant Virgo Cluster galaxy NGC 4472 (M49) which were detected with narrow-band imaging, we have made follow-up spectroscopic observations using the WYFFOS/AUTOFIB2 multifiber spectrograph on the 4.2-m William Herschel Telescope. We present here the results of these observations, as well as an analysis of the galaxy's dynamics.

Halo dynamics of elliptical galaxies
Aaron J. Romanowsky
Bulletin of the American Astronomical Society, Volume 32, Number 2, 2000, p. 695 (16.01)
(presented at the 196th meeting of the American Astronomical Society in June 2000)
ADS | Press release
Citations: [1]

Progress on the understanding of the halo dynamics in elliptical galaxies has lagged behind that in spiral galaxies owing to the general lack of simple, easily-observable dynamical tracers in the former. But this situation is now being remedied with the routine observation of high-resolution stellar line-of-sight velocity distributions (LOSVDs) and of discrete object velocities at large radii (globular clusters and planetary nebulae), in combination with improved dynamical modeling techniques.
We investigate the dynamics of the giant elliptical galaxy M87. The data include higher-order moments of the stellar LOSVD, and the discrete velocities of over 200 globular clusters. These provide dynamical constraints out to 5 effective radii (40 kpc). Our spherical models allow for full freedom of variations in the orbital anisotropy; we introduce an extension to orbit-based modeling techniques that fits the discrete velocity data using maximum likelihood.
For this galaxy, we are able to strongly rule out a constant mass-to-light ratio. We find that the total mass falls off more slowly than r^-2 in the outer parts, suggesting that the core of the dark halo of the Virgo Cluster itself is being detected.

Dynamics of the Sgr A* cluster
Aaron J. Romanowsky & Christopher S. Kochanek
Dynamics of Star Clusters and the Milky Way, ASP Conference Series Vol. 228, 2001, ed. S. Deiters, B. Fuchs, A. Just, R. Spurzem, & R. Wielen (San Francisco: ASP), pp. 556-558
(presented at *2000: Dynamics of Star Clusters and the Milky Way in March 2000)
ADS1 | ADS2 | local gzipped postscript
Proper motions | Galactic Center in IR
Citations: [1]

We model the dynamics of the star cluster around Sgr A* using a spherical potential and an arbitrary two-integral distribution function in order to fully include the uncertainties created by orbital anisotropy. Given spatial and kinematical data for the stars, we find the best-fit parameters of the mass distribution, assuming a point mass embedded in a singular isothermal sphere. We also determine the distance to the Galactic center using statistical parallax.

Halo dynamics of M87
Aaron J. Romanowsky & Christopher S. Kochanek
XVth IAP Meeting, Dynamics of Galaxies: From the Early Universe to the Present, ASP Conference Series Vol. 197, 2000, ed. F. Combes, G. A. Mamon, & V. Charmandaris (San Francisco: ASP), pp. 401-402
(presented at Dynamics of Galaxies: from the Early Universe to the Present in July 1999)
ADS | local postscript?!
Citations: [1]

We examine the dynamics of the stars and globular clusters in the nearby giant galaxy M87 and constrain the mass distribution using all the available kinematical data, including higher-order moments of the stellar line-of-sight velocity distributions and the discrete velocities of more than 200 globular clusters. Employing a robust orbit modeling method that makes full use of all the information in the data, we rule out both a constant mass-to-light ratio model and a singular isothermal mass model. Our results imply the presence of a dark halo with a radial density profile that falls off more slowly than r^-2, suggesting that the potential of the Virgo Cluster is already dominant at r ~ 20 kpc.

Stellar dynamics at the Galactic center
Aaron J. Romanowsky & Christopher S. Kochanek
Bulletin of the American Astronomical Society, Volume 31, Number 1, 1999, p. 666 (123.01)
(presented at the 193rd meeting of the American Astronomical Society in January 1999)
ADS

We determine the dynamics of the stellar cluster around the Galactic center, Sgr A*. The data that we fit include the surface density distribution of the stars, along with discrete measurements of their radial velocities and proper motions. Our method nonparametrically finds the stellar distribution function in the combined spherical potential of the central black hole and the stellar cluster (which is assumed to have a 1/r² density profile). By including the uncertainties in the radially-varying stellar orbit anisotropy, this provides rigorous constraints on the mass of the black hole. Furthermore, the statistical parallax allows us to directly determine the distance to Sgr A*.

A global dynamical model of M87
Aaron J. Romanowsky
Galaxy Dynamics, A Rutgers Symposium, ASP Conference Series Vol. 182, 1999, ed. D. Merritt, J. A. Sellwood, & M. Valluri (San Francisco: ASP), pp. 158-159
(presented at the Rutgers University Conference on Galaxy Dynamics in August 1998)
ADS | local compressed postscript?!

We combine all of the available observational constraints to construct dynamical models of the giant galaxy M87, making particular use of the positions and velocities of the globular clusters, and of the higher-order moments of the stellar velocity profiles. By extending Schwarzschild's method of orbit modeling, we provide very general constraints on the mass distribution and the orbit structure of the galaxy.

Orbit modeling of the lensing galaxy in 0957+561
Aaron J. Romanowsky
Galactic Halos: A UC Santa Cruz Workshop, ASP Conference Series Vol. 136, 1998, ed. D. Zaritsky (San Francisco: ASP), pp. 317-319
(presented at the UC Santa Cruz Workshop on Galactic Halos in August 1997)
ADS | local postscript?!

The gravitational lens system 0957+561 can be used to determine the Hubble constant H₀ given an estimate of the lensing galaxy's halo velocity dispersion $\sigma_0$ , where $H_0 \propto \sigma_0^2$ . We employ orbit modeling to study the conversion of a measured central projected stellar velocity dispersion $\sigma_p$ to $\sigma_0$ . The model uncertainty is enormous, due to the freedom from the orbit anisotropy variation. Even with the added requirements that the velocity dispersion $\sigma_p(R)$ and fourth-order Gauss-Hermite velocity moment h₄(R) profiles are similar to those of nearby elliptical galaxies, the resulting uncertainty in H₀, while decreased, is still too large to be useful.

New projections of triaxiality
Aaron J. Romanowsky & Christopher S. Kochanek
The Second Stromlo Symposium: The Nature of Elliptical Galaxies, ASP Conference Series Vol. 116, 1997, ed. M. Arnaboldi, G. S. Da Costa & P. Saha (San Francisco: ASP), pp. 107-108
(presented at the Second Stromlo Symposium on the Nature of Elliptical Galaxies in August 1996)
ADS | local postscript?! | local HTML?!

An elliptical galaxy is considered to consist of a triaxial luminous stellar distribution embedded in a triaxial "dark" halo. Differences in the triaxialities lead to kinematic misalignments, X-ray isophote twists, and gravitational lens model misalignments. We have numerically projected the X-ray emission of a model stellar and halo potential and compared it to the luminous galaxy projection for a variety of viewing angles. The two projections generally show a small misalignment, with an X-ray isophote twist of ~ 5° occurring near the effective radius. The large misalignment observed in NGC 720 could not be well fit, suggesting that it requires an intrinsic misalignment of the halo and the galaxy.

Ultrashort-pulse reflectometry
C. W. Domier, N. C. Luhmann, Jr., A. E. Chou, W.-M. Zhang, and A. J. Romanowsky
Review of Scientific Instruments, Volume 66, Issue 1, January 1995, pp. 399-401
(presented at Tenth Topical Conference on High-Temperature Plasma Diagnostics on 9 May 1994)
Online

Time-of-flight radar diagnostics are envisaged as having great potential for determining electron density profiles in next generation tokamaks such as TPX and ITER. Ultrashort-pulse radar reflectometry is a promising new time-of-flight diagnostic capable of making instantaneous density profile determination utilizing a single source and a single set of measurements. A proof-of-principle eight channel system has been constructed for use on the CCT tokamak at UCLA, and has undergone extensive testing in the laboratory.

Last updated 31 January 2012